[Don't Merge] Replace the deprecated pkg_resources API [RHELDST-33715]

[dichn]

pkg_resources is deprecated as an API. Updated Source.__load_entrypoints to use importlib.metadata.entry_points, removing the need for pkg_resources.

🚫 Don't merge this until the primary python version bumping to 3.10+
This issue is to fix a deprecation warning

../../dev/python3.11/lib64/python3.11/site-packages/pushsource/_impl/source.py:8
  /home/dichen/dev/python3.11/lib64/python3.11/site-packages/pushsource/_impl/source.py:8: DeprecationWarning: pkg_resources is deprecated as an API. See https://setuptools.pypa.io/en/latest/pkg_resources.html
    import pkg_resources 

But the alternative solution importlib.metadata was added only in Python 3.8, and its behavior differs between Python 3.8–3.9 and Python 3.10+.

# Python 3.10+: entry_points(group=...)
eps = entry_points(group="pushsource")
# Python 3.8–3.9: entry_points() returns dict-like
eps = entry_points().get("pushsource", [])

In order to make the replacement clean, let's include this change after the primary python version bumping to 3.10+.

[dichn]

Close this PR due to lacking backwards compatibility. It can be re-opened if necessary.