Fix SetuptoolsDeprecationWarning: \project.license\ as a TOML table is deprecated

[dweindl]

Fixes #2695:

          ********************************************************************************
          Please use a simple string containing a SPDX expression for `project.license`. You can also use `project.license-files`. (Both options available on setuptools>=77.0.0).

          By 2026-Feb-18, you need to update your project and remove deprecated calls
          or your builds will no longer be supported.

          See https://packaging.python.org/en/latest/guides/writing-pyproject-toml/#license for details.
          ********************************************************************************

and

          ********************************************************************************
          Please consider removing the following classifiers in favor of a SPDX license expression:

          License :: OSI Approved :: BSD License

          See https://packaging.python.org/en/latest/guides/writing-pyproject-toml/#license for details.
          ********************************************************************************

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 77.92%. Comparing base (ff1ee23) to head (bed56fa).

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #2697      +/-   ##
===========================================
- Coverage    77.93%   77.92%   -0.02%     
===========================================
  Files          326      326              
  Lines        22994    22994              
  Branches      1479     1479              
===========================================
- Hits         17920    17917       -3     
- Misses        5063     5066       +3     
  Partials        11       11              

Flag	Coverage Δ
cpp	74.76% <ø> (-0.02%)	⬇️
cpp_python	33.58% <ø> (ø)
petab	39.44% <ø> (ø)
python	72.96% <ø> (-0.01%)	⬇️

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see 3 files with indirect coverage changes

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