Tests: get tox py36 tests working again

tox dependency virtualenv dropped support for creating
Python 3.6 environments in a minor release. [1]

Announce deprecation of Anymail Python 3.6 support.
Until that can take effect, pin an older version of
virtualenv that still works for tox py36 testenv.

[1]: See https://github.com/pypa/virtualenv/pull/2548#issuecomment-1527278210 et seq

.github/workflows/test.yml

@@ -57,7 +57,8 @@ jobs:
         run: |
           set -x
           python -VV
-          python -m pip install 'tox<4'
+          # Must pin virtualenv for tox py36 testenv:
+          python -m pip install 'tox<4' 'virtualenv<20.22.0'
           python -m tox --version
       - name: Test ${{ matrix.tox.name }}
         run: |