Fix running tests under Python 2

As per #1961.

P.S. Using `contains(['2.7', 'pypy2'], matrix.python-version)` does not
work even though
https://help.github.com/en/actions/automating-your-workflow-with-github-actions/contexts-and-expression-syntax-for-github-actions#contains
promises arrays to be supported.

.github/workflows/python-tests.yml

@@ -76,11 +76,13 @@ jobs:
 
     - name: 'Initialize tox envs: ${{ matrix.env.TOXENV }}'
       run: |
+        ${{ startsWith(matrix.os, 'windows-') && 'setx TOXENV ' || 'export TOXENV=' }}${{ ('2.7' == matrix.python-version || 'pypy2' == matrix.python-version) && 'py27' || '$TOXENV' }}
         python -m tox --parallel auto --notest --skip-missing-interpreters false
       env: ${{ matrix.env }}
     - name: Test with tox
       run: |
         ${{ startsWith(matrix.os, 'windows-') && 'setx NETWORK_REQUIRED ' || 'export NETWORK_REQUIRED=' }}1
+        ${{ startsWith(matrix.os, 'windows-') && 'setx TOXENV ' || 'export TOXENV=' }}${{ ('2.7' == matrix.python-version || 'pypy2' == matrix.python-version) && 'py27' || '$TOXENV' }}
         python -m tox \
         --parallel 0 \
         -- \