pipenv: handle installs with missing system deps

Raise a `DependencyFileNotResolvable` error when pipenv fails to install
dependencies require missing system level dependenceis.

The error message parsing is pretty gnarly as the only reference of the
offending dependency is in a tmp build path in the error message.

In the test case pipenv failed to install `rtree` because it requires a
system level dependency: `libspatialindex`.

We don't want to install this in our ever growning dockerfile so attempt
to explain the error to users.

The underlying cause might be obscured by this open issue in pipenv:
pypa/pipenv#2791

Similarly, there's an open issue to fix the system requirement in rtree:
Toblerity/rtree#147

This is the error pipenv raises:

```
pipenv.patched.notpip._internal.exceptions.InstallationError: Command "python setup.py egg_info" failed with error code 1 in /tmp/tmp8jk2atvgbuild/rtree/
File "/usr/local/.pyenv/versions/3.9.0/lib/python3.9/site-packages/pipenv/resolver.py", line 126, in <module>
    main()
  File "/usr/local/.pyenv/versions/3.9.0/lib/python3.9/site-packages/pipenv/resolver.py", line 118, in main
    _main(parsed.pre, parsed.clear, parsed.verbose, parsed.system,
  File "/usr/local/.pyenv/versions/3.9.0/lib/python3.9/site-packages/pipenv/resolver.py", line 78, in _main
    results = resolve(
  File "/usr/local/.pyenv/versions/3.9.0/lib/python3.9/site-packages/pipenv/resolver.py", line 61, in resolve
    return resolve_deps(
  File "/usr/local/.pyenv/versions/3.9.0/lib/python3.9/site-packages/pipenv/utils.py", line 718, in resolve_deps
    resolved_tree, hashes, markers_lookup, resolver = actually_resolve_deps(
  File "/usr/local/.pyenv/versions/3.9.0/lib/python3.9/site-packages/pipenv/utils.py", line 480, in actually_resolve_deps
    resolved_tree = resolver.resolve()
  File "/usr/local/.pyenv/versions/3.9.0/lib/python3.9/site-packages/pipenv/utils.py", line 385, in resolve
    results = self.resolver.resolve(max_rounds=environments.PIPENV_MAX_ROUNDS)
  File "/usr/local/.pyenv/versions/3.9.0/lib/python3.9/site-packages/pipenv/patched/piptools/resolver.py", line 102, in resolve
    has_changed, best_matches = self._resolve_one_round()
  File "/usr/local/.pyenv/versions/3.9.0/lib/python3.9/site-packages/pipenv/patched/piptools/resolver.py", line 206, in _resolve_one_round
    for dep in self._iter_dependencies(best_match):
  File "/usr/local/.pyenv/versions/3.9.0/lib/python3.9/site-packages/pipenv/patched/piptools/resolver.py", line 301, in _iter_dependencies
    dependencies = self.repository.get_dependencies(ireq)
  File "/usr/local/.pyenv/versions/3.9.0/lib/python3.9/site-packages/pipenv/patched/piptools/repositories/pypi.py", line 234, in get_dependencies
    legacy_results = self.get_legacy_dependencies(ireq)
  File "/usr/local/.pyenv/versions/3.9.0/lib/python3.9/site-packages/pipenv/patched/piptools/repositories/pypi.py", line 426, in get_legacy_dependencies
    results, ireq = self.resolve_reqs(download_dir, ireq, wheel_cache)
  File "/usr/local/.pyenv/versions/3.9.0/lib/python3.9/site-packages/pipenv/patched/piptools/repositories/pypi.py", line 297, in resolve_reqs
    results = resolver._resolve_one(reqset, ireq)
  File "/usr/local/.pyenv/versions/3.9.0/lib/python3.9/site-packages/pipenv/patched/notpip/_internal/resolve.py", line 260, in _resolve_one
    abstract_dist = self._get_abstract_dist_for(req_to_install)
  File "/usr/local/.pyenv/versions/3.9.0/lib/python3.9/site-packages/pipenv/patched/notpip/_internal/resolve.py", line 211, in _get_abstract_dist_for
    abstract_dist = self.preparer.prepare_linked_requirement(
  File "/usr/local/.pyenv/versions/3.9.0/lib/python3.9/site-packages/pipenv/patched/notpip/_internal/operations/prepare.py", line 294, in prepare_linked_
    abstract_dist.prep_for_dist(finder, self.build_isolation)
  File "/usr/local/.pyenv/versions/3.9.0/lib/python3.9/site-packages/pipenv/patched/notpip/_internal/operations/prepare.py", line 127, in prep_for_dist
    self.req.run_egg_info()
  File "/usr/local/.pyenv/versions/3.9.0/lib/python3.9/site-packages/pipenv/patched/notpip/_internal/req/req_install.py", line 470, in run_egg_info
    call_subprocess(
  File "/usr/local/.pyenv/versions/3.9.0/lib/python3.9/site-packages/pipenv/patched/notpip/_internal/utils/misc.py", line 703, in call_subprocess
    raise InstallationError(
pipenv.patched.notpip._internal.exceptions.InstallationError: Command "python setup.py egg_info" failed with error code 1 in /tmp/tmp8jk2atvgbuild/rtree/
```

python/lib/dependabot/python/update_checker/pipenv_version_resolver.rb

@@ -39,6 +39,12 @@ class PipenvVersionResolver
         UNSUPPORTED_DEP_REGEX =
           /"python setup\.py egg_info".*(?:#{UNSUPPORTED_DEPS.join("|")})/.
           freeze
+        PIPENV_INSTALLATION_ERROR = "pipenv.patched.notpip._internal."\
+                                    "exceptions.InstallationError: "\
+                                    "Command \"python setup.py egg_info\" "\
+                                    "failed with error code 1 in"
+        PIPENV_INSTALLATION_ERROR_REGEX =
+          %r{#{Regexp.quote(PIPENV_INSTALLATION_ERROR)}.+/(?<name>.+)/$}.freeze
 
         attr_reader :dependency, :dependency_files, :credentials
 
@@ -169,7 +175,6 @@ def handle_pipenv_errors(error)
             return if error.message.match?(/#{Regexp.quote(dependency.name)}/i)
           end
 
-          puts error.message
           if error.message.match?(GIT_DEPENDENCY_UNREACHABLE_REGEX)
             url = error.message.match(GIT_DEPENDENCY_UNREACHABLE_REGEX).
                   named_captures.fetch("url")
@@ -232,6 +237,10 @@ def handle_pipenv_errors_resolving_original_reqs(error)
             raise DependencyFileNotResolvable, msg
           end
 
+          # NOTE: Pipenv masks the actualy error, see this issue for updates:
+          # https://github.com/pypa/pipenv/issues/2791
+          handle_pipenv_installation_error(error.message) if error.message.match?(PIPENV_INSTALLATION_ERROR_REGEX)
+
           # Raise an unhandled error, as this could be a problem with
           # Dependabot's infrastructure, rather than the Pipfile
           raise
@@ -257,6 +266,19 @@ def clean_error_message(message)
           msg.gsub(/http.*?(?=\s)/, "<redacted>")
         end
 
+        def handle_pipenv_installation_error(error_message)
+          # Find the dependency that's causing resolution to fail
+          dependency_name = error_message.match(PIPENV_INSTALLATION_ERROR_REGEX).named_captures["name"]
+          raise unless dependency_name
+
+          msg = "Pipenv failed to install \"#{dependency_name}\". This could be caused by missing system "\
+                "dependencies that can't be installed by Dependabot or required installation flags.\n\n"\
+                "Error output from running \"pipenv lock\":\n"\
+                "#{clean_error_message(error_message)}"
+
+          raise DependencyFileNotResolvable, msg
+        end
+
         def write_temporary_dependency_files(updated_req: nil,
                                              update_pipfile: true)
           dependency_files.each do |file|

python/spec/dependabot/python/update_checker/pipenv_version_resolver_spec.rb

@@ -469,6 +469,35 @@
           end
       end
     end
+
+    context "with a missing system libary" do
+      # NOTE: Attempt to update an unrelated dependency (tensorflow) to cause
+      # resolution to fail for rtree which has a system dependency on
+      # libspatialindex which isn't installed in dependabot-core's Dockerfile.
+      let(:dependency_files) do
+        project_dependency_files("pipenv/missing-system-library")
+      end
+      let(:updated_requirement) { "==2.3.1" }
+      let(:dependency_name) { "tensorflow" }
+      let(:dependency_version) { "2.1.0" }
+      let(:dependency_requirements) do
+        [{
+          file: "Pipfile",
+          requirement: "==2.1.0",
+          groups: ["default"],
+          source: nil
+        }]
+      end
+
+      it "raises a helpful error" do
+        expect { subject }.
+          to raise_error(Dependabot::DependencyFileNotResolvable) do |error|
+            expect(error.message).to include(
+              "Pipenv failed to install \"rtree\""
+            )
+          end
+      end
+    end
   end
 
   describe "#resolvable?" do

python/spec/fixtures/projects/pipenv/missing-system-library/Pipfile

@@ -0,0 +1,8 @@
+[[source]]
+url = "https://pypi.org/simple"
+verify_ssl = true
+name = "pypi"
+
+[packages]
+rtree = "==0.9.3"
+tensorflow = "==2.1.0"