Use Conda Python headers

[DimitriPapadopoulos]

Fix CI failure. Convince Meson to pick Python headers installed by Conda, not system-wide headers.

TODO:

• Unit tests and/or doctests in docstrings
• Tests pass locally
• Docstrings and API docs for any new/modified user-facing classes and functions
• Changes documented in docs/release.rst
• Docs build locally
• GitHub Actions CI passes
• Test coverage to 100% (Codecov passes)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.22%. Comparing base (1fdde70) to head (17c63a0).

Additional details and impacted files

@@             Coverage Diff             @@
##              main     #839      +/-   ##
===========================================
- Coverage   100.00%   98.22%   -1.78%     
===========================================
  Files           27       27              
  Lines          901      901              
===========================================
- Hits           901      885      -16     
- Misses           0       16      +16     

see 2 files with indirect coverage changes

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