Remove `pkg_resources` from codebase

[stes]

Current tests raise the following warning

cebra/integrations/sklearn/cebra.py:31
  /home/runner/work/CEBRA/CEBRA/cebra/integrations/sklearn/cebra.py:31: UserWarning: pkg_resources is deprecated as an API. See https://setuptools.pypa.io/en/latest/pkg_resources.html. The pkg_resources package is slated for removal as early as 2025-11-30. Refrain from using this package or pin to Setuptools<81.
    import pkg_resources

needs to be done before end of November 2025.

[vovw]

#272 resolves this