Test against Python 3.15 on Linux

[Jens Hedegaard Nielsen (jenshnielsen)]

• Add Python 3.15 to the Linux pytest matrix (allow prereleases until 3.15.0 final is in the actions/python-versions manifest).
• Build h5py from source on 3.15 by installing the HDF5 headers since h5py has no cp315 wheels yet.
• Skip the zhinst-qcodes test dependency on 3.15 since zhinst-core has no cp315 wheels, and ignore missing zhinst imports in mypy and pyright.
• Bump pyarrow to 26.0.0 and markupsafe to 3.0.4 which ship cp315 wheels.
• Add the Python 3.15 classifier.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 72.37%. Comparing base (09ec44d) to head (8ca6a6a).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #8581   +/-   ##
=======================================
  Coverage   72.37%   72.37%           
=======================================
  Files         307      307           
  Lines       32397    32397           
=======================================
  Hits        23447    23447           
  Misses       8950     8950           

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