DEP: drop undeclared runtime dependency on setuptools

[neutrinoceros]

Discovered while trying to remove setuptools as a dependency in glue-core
Note that the problem is evident when testing mpl-scatter-density in a fresh env that doesn't have setuptools, e.g., using uv venv

uv venv
uv pip install -e .
source .venv/bin/activate
python -c "import mpl_scatter_density"

Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "/Users/clm/dev/astropy-project/deps/mpl-scatter-density/mpl_scatter_density/__init__.py", line 4, in <module>
    from pkg_resources import get_distribution, DistributionNotFound
ModuleNotFoundError: No module named 'pkg_resources'

[neutrinoceros]

@astrofrog can you have a look ?

[neutrinoceros]

@astrofrog I'm also fixing CI failures in #44

[astrofrog]

Thanks, feel free to also drop old Python versions if it simplifies things. 3.9 or later would be fine.

[neutrinoceros]

Happy to do it as a follow up. Doing it here would actually make the patch bigger.