Add stab at Python 3.9 built test job

Have to reinstall local virtualenv to new 3.9

Otherwise, the pip install of tox &c. pulls the PyPI version
of virtualenv for use in creating the tox envs.

azure-pipelines.yml

@@ -24,6 +24,10 @@ variables:
   PYTEST_ADDOPTS: "-v -v -ra --showlocals"
   PYTEST_XDIST_PROC_NR: 'auto'
   CI_RUN: 'yes'
+  PY39_BUILD: "$(Build.Repository.LocalPath)/py39-build"
+  PY39_INSTALL: "$(Build.Repository.LocalPath)/python3.9"
+  PY39_VERSION: "3.9.0a1"
+  PY39_SHORTVERSION: "3.9.0"
 
 jobs:
 - template: run-tox-env.yml@tox
@@ -57,6 +61,23 @@ jobs:
       with_toxenv: 'coverage' # generate .tox/.coverage, .tox/coverage.xml after test run
       for_envs: [py37, py36, py35, py34, py27]
 
+- job: py39_linux
+  pool:
+    vmImage: 'Ubuntu-16.04'
+  steps:
+    - script: 'mkdir "$(PY39_BUILD)" && wget -P "$(PY39_BUILD)" https://www.python.org/ftp/python/$(PY39_SHORTVERSION)/Python-$(PY39_VERSION).tgz'
+      displayName: "Download Python $(PY39_VERSION)"
+    - script: 'cd "$(PY39_BUILD)" && tar xf "Python-$(PY39_VERSION).tgz"'
+      displayName: "Extract tarball"
+    - script: 'cd "$(PY39_BUILD)/Python-$(PY39_VERSION)" && ./configure --prefix="$(PY39_INSTALL)" && make && make install'
+      displayName: "Build Python"
+    - script: '$(PY39_INSTALL)/bin/python3.9 -m pip install -U pip setuptools wheel tox'
+      displayName: "Update Python 3.9 pip environment"
+    - script: '$(PY39_INSTALL)/bin/python3.9 -m pip install -e .'
+      displayName: "Reset virtualenv version to local"
+    - script: '$(PY39_INSTALL)/bin/python3.9 -m tox -e py39'
+      displayName: "Run tox tests"
+
 - ${{ if startsWith(variables['Build.SourceBranch'], 'refs/tags/') }}:
   - template: publish-pypi.yml@tox
     parameters: