Remove runtime setuptools dependency

[kajaste]

Prefer importlib.metadata core library which is available on Python >= 3.8

This gets rid of setuptools dependency for stopit users. Recent setuptools versions output a massive deprecation warning about pkg_resources usage whenever stopit is imported:

DeprecationWarning: pkg_resources is deprecated as an API. See https://setuptools.pypa.io/en/latest/pkg_resources.html)

Additionally, not all Python environments have setuptools these days, which makes import stopit fail.

Also: update supported Python versions in setup.py with versions I have successfully used stopit on

PS. Thank you so much for the awesome library. It has helped me through several hardships over the years

[peterjc]

I also was troubled by this deprecation warning (triggered via snakemake), and identified the same pkg_resources.get_distribution(__name__).version change to fix it. Applying this fix and doing a point release would be appreciated.

(As an aside, if you are looking for a volunteer to help with or take over maintenance of the package, reaching out to the snakemake community seems sensible.)