ci: add Python version to cache key

[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

https://github.com/fpgmaas/deptry/actions/runs/3320886881/jobs/5487809759#step:4:20 highlights that we are currently running tox under Python 3.10 regardless of the Python version sent as an input, because we share the cache key regardless of the version.

[codecov-commenter]

Codecov Report

Merging #173 (06a634b) into main (6f54023) will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff          @@
##            main    #173   +/-   ##
=====================================
  Coverage   95.1%   95.1%           
=====================================
  Files         28      28           
  Lines        895     895           
  Branches     146     136   -10     
=====================================
  Hits         852     852           
  Misses        28      28           
  Partials      15      15           

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