[MISC] Increase coverage

[dragomirp]

• Increase unit test coverage
• Make tests run on python 3.12+

Checklist

• I have added or updated any relevant documentation.
• I have cleaned any remaining cloud resources from my accounts.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 40.64%. Comparing base (d35338f) to head (29663a4).
⚠️ Report is 1 commits behind head on 16/edge.

❌ Your project status has failed because the head coverage (40.64%) is below the target coverage (70.00%). You can increase the head coverage or adjust the target coverage.

Additional details and impacted files

@@             Coverage Diff             @@
##           16/edge       #9      +/-   ##
===========================================
+ Coverage    32.21%   40.64%   +8.42%     
===========================================
  Files            3        3              
  Lines          776      780       +4     
  Branches        91       92       +1     
===========================================
+ Hits           250      317      +67     
+ Misses         518      448      -70     
- Partials         8       15       +7     

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[dragomirp]

Linting for the tests only. We have to keep 3.8 compatibility for PGB VM charm, but there's no need to also keep 3.8 support for the tests.

[dragomirp]

No need to exclude as we bump coverage.

[taurus-forever]

LGTM