test: GitHub Actions enable python-version: 3.5

And upgrade actions/checkout@v1 to v2

Author: cclauss

.github/workflows/Python_tests.yml

@@ -1,4 +1,3 @@
-# TODO: Line 15, enable python-version: 3.5
 # TODO: Line 36, enable pytest --doctest-modules
 
 name: Python_tests
@@ -11,9 +10,9 @@ jobs:
       max-parallel: 15
       matrix:
         os: [macos-latest, ubuntu-latest, windows-latest]
-        python-version: [2.7, 3.6, 3.7, 3.8]  # 3.5,
+        python-version: [2.7, 3.5, 3.6, 3.7, 3.8]
     steps:
-      - uses: actions/checkout@v1
+      - uses: actions/checkout@v2
       - name: Set up Python ${{ matrix.python-version }}
         uses: actions/setup-python@v1
         with: