Run CI for Python3.9 (#4647)

* Run CI for Python3.9, skips hdfs tests for 3.9

Also, `dvc[hdfs]` install will be a no-op till we have
a pyarrow wheel for 3.9.

Note that this does not add an entry to the classifier
claiming it supports 3.9, that will be added when 3.9
gets released (or, wait for pyarrow to be released).

* Skip HDFS related tests on Python3.9

Also, fix the python version regex for `3.9.0b5` like versions.

* Exclude python3.9 for Windows

libxml does not have wheels for Python39.
We do compile them on Linux and Mac

* Update tests/unit/test_info.py

* Use realpath for gitignore

Windows tests for Python3.8 was failing
intermittently.

Note: Also removed the realpath wrap around self.root_dir as it's already a
realpath when `Git` is instantiated.

Co-authored-by: Ruslan Kuprieiev <[email protected]>

.github/workflows/tests.yaml

@@ -28,16 +28,21 @@ jobs:
       fail-fast: false
       matrix:
         os: [ubuntu-18.04, windows-2019, macos-10.15]
-        pyv: [3.6, 3.7, 3.8]
+        pyv: [3.6, 3.7, 3.8, "3.9-dev"]
         exclude:
           - os: windows-2019
-            pyv: 3.8
+            pyv: "3.9-dev"
     steps:
     - uses: actions/[email protected]
     - name: Set up Python
       uses: actions/[email protected]
       with:
         python-version: ${{ matrix.pyv }}
+    - name: Install libxml2 and libxslt
+      if: matrix.pyv == '3.9-dev' && matrix.os == 'ubuntu-18.04'
+      # need to install for building lxml, a dependency of webdavclient3
+      run: |
+        sudo apt install libxml2 libxslt-dev
     - name: install
       run: |
         pip install ".[all,tests]"
@@ -46,12 +51,10 @@ jobs:
         git config --global user.email "[email protected]"
         git config --global user.name "DVC Tester"
     - name: run tests
-      run: python -m tests --cov-report=xml
+      run: python -m tests -ra --cov-report=xml
     - name: upload coverage report
       if: github.event.name == 'push'
       uses: codecov/[email protected]
       with:
         file: ./coverage.xml
         fail_ci_if_error: true
-
-