fix error with DistrubutionNotFound and switch to PackageNotFoundError

[ASKabalan]

This pull request updates how the package version is retrieved in jax_cosmo/__init__.py to use the modern importlib.metadata module instead of the deprecated pkg_resources. This change improves compatibility with newer Python versions and avoids unnecessary dependencies.

• Modernized version retrieval:
  • Replaced pkg_resources.get_distribution and DistributionNotFound with importlib.metadata.version and PackageNotFoundError in jax_cosmo/__init__.py.

This warning is now an error on python 3.10

Traceback:
/opt/hostedtoolcache/Python/3.10.19/x64/lib/python3.10/importlib/__init__.py:126: in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
test_spherical_painting_methods.py:25: in <module>
    import jax_cosmo as jc
/opt/hostedtoolcache/Python/3.10.19/x64/lib/python3.10/site-packages/jax_cosmo/__init__.py:2: in <module>
    from pkg_resources import DistributionNotFound
E   ModuleNotFoundError: No module named 'pkg_resources'

Which blocs JaxPM CI => https://github.com/DifferentiableUniverseInitiative/JaxPM/actions/runs/22356291638/job/64696767811?pr=54

[ASKabalan]

Please merge before #145