update package metadata

[orbeckst]

See https://github.com/MDAnalysis/MDAnalysisData/actions/runs/23919873212/job/69763201067

• license description: use SPDX
• deprecated codec.open in setup.py
• fix importable packages

Importable packages issue:

/tmp/build-env-yhrk7fq6/lib/python3.14/site-packages/setuptools/command/build_py.py:215: _Warning: Package 'MDAnalysisData.descr' is absent from the `packages` configuration.
!!

        ********************************************************************************
        ############################
        # Package would be ignored #
        ############################
        Python recognizes 'MDAnalysisData.descr' as an importable package[^1],
        but it is absent from setuptools' `packages` configuration.

        This leads to an ambiguous overall configuration. If you want to distribute this
        package, please make sure that 'MDAnalysisData.descr' is explicitly added
        to the `packages` configuration field.

        Alternatively, you can also rely on setuptools' discovery methods
        (for example by using `find_namespace_packages(...)`/`find_namespace:`
        instead of `find_packages(...)`/`find:`).

        You can read more about "package discovery" on setuptools documentation page:

        - https://setuptools.pypa.io/en/latest/userguide/package_discovery.html

        If you don't want 'MDAnalysisData.descr' to be distributed and are
        already explicitly excluding 'MDAnalysisData.descr' via
        `find_namespace_packages(...)/find_namespace` or `find_packages(...)/find`,
        you can try to use `exclude_package_data`, or `include-package-data=False` in
        combination with a more fine grained `package-data` configuration.

        You can read more about "package data files" on setuptools documentation page:

        - https://setuptools.pypa.io/en/latest/userguide/datafiles.html


        [^1]: For Python, any directory (with suitable naming) can be imported,
              even if it does not contain any `.py` files.
              On the other hand, currently there is no concept of package data
              directory, all directories are treated like packages.
        ********************************************************************************

!!
  check.warn(importable)
creating build/lib/MDAnalysisData/descr