ci: fix tox workflow

[mkniewallner]

PR Checklist

• A description of the changes is added to the description of this PR.
• If there is a related issue, make sure it is linked to this PR.
• If you've fixed a bug or added code that should be tested, add tests!
• Documentation in docs is updated

Description of changes

tox workflow is currently broken because we don't pin a specific version of tox-gh-actions, and a new major version was released yesterday (https://github.com/ymyzk/tox-gh-actions/releases/tag/v3.0.0).

This made us automatically pick up tox>=4, which removed whitelist_externals option in favour of allowlist_externals (https://github.com/tox-dev/tox/releases/tag/4.0.0rc4).

So the PR updates the parameter used, and updates the installation of tox-gh-actions to pin a specific version, as well as hinting Renovate on it, so we can still benefit from regular updates while facilitating build reproducibility.

It also removes the installation of tox which is useless, as tox-gh-actions already depend on one specific major version of tox. We can even see in the previously working build that we installed 4.x.y, but then went back to 3.x.y because tox-gh-actions<3 didn't accept tox>=4.

[codecov-commenter]

Codecov Report

Merging #241 (945d285) into main (7f4dc05) will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff          @@
##            main    #241   +/-   ##
=====================================
  Coverage   95.7%   95.7%           
=====================================
  Files         32      32           
  Lines        914     914           
  Branches     203     203           
=====================================
  Hits         875     875           
  Misses        26      26           
  Partials      13      13           

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