Change packaging to use setuptools declarative config in setup.cfg

[michael-k]

For the same change in Django, see PR django/django#12013 and issue 30948.

I'm not a packaging expert. According to my tests, the following blocks should lead to the same result:

# setup.py
setup(
    packages=find_packages() + ["twisted.plugins"],
)

# setup.cfg
[options]
packages = find_namespace:

[options.packages.find]
exclude =
    tests

I tried to convert setup.py 1:1 to setup.cfg with two exceptions:

1. python_requires = >=3.5 is new (Django also has a python_requires)
2. Classifier Programming Language :: Python :: 3 :: Only is new and also present in Django.