Upgrade GHA actions using deprecated env mechanism

.github/workflows/python-tests.yml

@@ -62,7 +62,7 @@ jobs:
       with:
         python-version: ${{ env.PYTHON_VERSION }}
     - name: Set up Python ${{ env.PYTHON_VERSION }}
-      uses: actions/setup-python@v2.1.1
+      uses: actions/setup-python@v2
       if: >-
         !fromJSON(env.USE_DEADSNAKES) && true || false
       with:
@@ -77,7 +77,7 @@ jobs:
       run: >-
         python -m sysconfig
     - name: Pip cache
-      uses: actions/cache@v1
+      uses: actions/cache@v2
       with:
         path: ~/.cache/pip
         key: ${{ runner.os }}-pip-${{ hashFiles('setup.cfg') }}