Python: Honour --strip-extras flag of pip-compile

The `--strip-extras` flag of `pip-compile` is useful to generate
contstraints files (instead of the more traditional requirements files)
from some `constraints.in` file.

Constraints files can't have extras markers on the packages (this
wouldn't make much sense indeed), hence they need to be stripped. Since
`pip-tools` 6.2.0 this is supported by using the `--strip-extras` flag.

This commit adds this as a 'recognized' flag: if it's found in an
existing `foo.txt` file, it will be passed to `pip-compile` when some
`foo.in` file is updated.

See: dependabot#3974 (comment)
See: dependabot#3974 (comment)
See: jazzband/pip-tools#1300 (comment)

python/lib/dependabot/python/file_updater/pip_compile_file_updater.rb

@@ -430,6 +430,9 @@ def pip_compile_options_from_compiled_file(requirements_file)
           options << "--no-header" unless requirements_file.content.include?("autogenerated by pip-c")
 
           options << "--pre" if requirements_file.content.include?("--pre")
+
+          options << "--strip-extras" if requirements_file.content.include?("--strip-extras")
+
           options
         end
 

python/spec/dependabot/python/file_updater/pip_compile_file_updater_spec.rb

@@ -505,5 +505,21 @@
         end
       end
     end
+
+    context "with stripped extras" do
+      let(:manifest_fixture_name) { "strip_extras.in" }
+      let(:generated_fixture_name) { "pip_compile_strip_extras.txt" }
+      let(:dependency_name) { "cachecontrol" }
+      let(:dependency_version) { "0.12.10" }
+      let(:dependency_previous_version) { "0.12.9" }
+
+      it "doesn't add an extras annotation on cachecontrol" do
+        expect(updated_files.count).to eq(1)
+        expect(updated_files.first.content).to include("--strip-extras")
+        expect(updated_files.first.content).to include("cachecontrol==0.12.10")
+        expect(updated_files.first.content).
+          to_not include("cachecontrol[filecache]==")
+      end
+    end
   end
 end

python/spec/fixtures/pip_compile_files/strip_extras.in

@@ -0,0 +1 @@
+cachecontrol[filecache]

python/spec/fixtures/requirements/pip_compile_strip_extras.txt

@@ -0,0 +1,22 @@
+#
+# This file is autogenerated by pip-compile with python 3.10
+# To update, run:
+#
+#    pip-compile --strip-extras --output-file=constraints.txt constraints.in
+#
+cachecontrol==0.12.10
+    # via -r constraints.in
+certifi==2021.10.8
+    # via requests
+charset-normalizer==2.0.7
+    # via requests
+idna==3.3
+    # via requests
+lockfile==0.12.2
+    # via cachecontrol
+msgpack==1.0.2
+    # via cachecontrol
+requests==2.26.0
+    # via cachecontrol
+urllib3==1.26.7
+    # via requests