FEAT: Add support and tests for Python 3.12

[sblauth]

This PR adds support for Python 3.12 for cashocs.

Closes #325

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 91.29%. Comparing base (16d6560) to head (6d1dbaf). Report is 9 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #324      +/-   ##
==========================================
- Coverage   91.29%   91.29%   -0.00%     
==========================================
  Files          86       86              
  Lines        7517     7516       -1     
==========================================
- Hits         6862     6861       -1     
  Misses        655      655              

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[sblauth]

It seems as if Python 3.12 is (from the FEniCS side) only supported with PETSc 3.20 - which is at the moment not compatible with cashocs, see #320

In case this incompatibility is resolved, there should be nothing standing in the way of using cashocs with Python 3.12

[sblauth]

See here https://gitlab.com/petsc/petsc/-/merge_requests/6976 for the corresponding issue in PETSc's Gitlab