[MAINTENANCE] migrate from pkg_resources to importlib.metadata to drop setuptools < 82 pin

[joda9]

Context

#599 revealed that setuptools >= 82 removes pkg_resources, requiring
us to pin setuptools < 82.0.0 as a workaround. This is a temporary fix —
pinning setuptools blocks security patches and future setuptools improvements.

Goal

Remove the setuptools < 82.0.0 pin by eliminating all pkg_resources
usage from eDisGo and its dependencies.

Steps

1. Audit eDisGo's own codebase for any pkg_resources usage (currently
   none found, but should be confirmed in CI).

2. Audit direct dependencies for pkg_resources usage:

   • Run grep -r "import pkg_resources" $(pip show <dep> -f | ...) for
     each dependency in setup.py.
   • Candidates to check: egoio, saio, workalendar, demandlib,
     descartes, pypower, pypsa, oedialect.

3. For each offending dependency:

   • Open an upstream issue / PR to migrate from pkg_resources to
     importlib.metadata (Python 3.9+ standard library).
   • Or update the version pin in setup.py once a fixed release is
     available.

4. Once all dependencies are clean, remove "setuptools < 82.0.0"
   from setup.py.

References

• setuptools changelog: removal of pkg_resources in v82
• Migration guide: https://setuptools.pypa.io/en/latest/pkg_resources.html
• Workaround: [BUG] ImportError: pkg_resources missing with setuptools>=82 #599