Tests failing for tox targeting Python 3.9.

[Mitja O]

As noticed in #396, the latest attempts to run test-cpython3.9 GitHub Action fails, meanwhile no code changes could trigger the error. This seems to be related to updated dependencies of tox as installed during the Install dependencies step in test-cpython3.9.

Currently passing commit on master branch has the following dependencies installed:

Successfully installed:
backports.entry-points-selectable-1.1.0
distlib-0.3.3
filelock-3.2.0
packaging-21.0
platformdirs-2.4.0
pluggy-1.0.0
py-1.10.0
pyparsing-2.4.7
six-1.16.0
toml-0.10.2
tox-3.24.4
virtualenv-20.8.1

While the failing run on the same commit on a fork of master branch has the following installations:

Successfully installed:
backports.entry-points-selectable-1.1.1 # ( backports.entry-points-selectable-1.1.0 in previous run! )
distlib-0.3.3
filelock-3.4.0 # ( filelock-3.2.0 in previous run! )
packaging-21.2 # ( packaging-21.0 in previous run! )
platformdirs-2.4.0
pluggy-1.0.0
py-1.11.0 # ( py-1.11.0 in previous run! )
pyparsing-2.4.7
six-1.16.0
toml-0.10.2
tox-3.24.4
virtualenv-20.10.0 # ( virtualenv-20.8.1 in previous run! )

My suspicion is that the failures are related to one of the package updates. Any additional insights are welcome.

Proposed actions:

• Pin tox dependencies to the working version
• Investigate which of the changes cause the issue

[Mitja O]

As a temporary step to resolve the CI/CD errors and unblock the pull requests, disabling the tests for 3.9 with #407.
(with a temporary intention :))