Build meta: fixes and cleanups

Fix test command

support `setup_requires` in setup.cfg

python 3 bdist_egg --exclude-source-files __pycache__ issue

- cleanup test: we're not installing, so no need to override the user site,
  or skip the test when run with a virtual environment
- use pytest support for capturing output (`context.quiet` does not work
  with Python 2), and check the output

When using Python 2, `python2 -m unittest` is not equivalent to
`python2 -m unittest discover`.

This code is a bit ugly, but it's also been tested with the pip test suite
It's not the best solution long term (the best solution is to get the egg_info
directory directly from egg_info), but it works for now and avoids
technical risk.

Include license file in the generated wheel package

The wheel package format supports including the license file. This is
done using the [metadata] section in the setup.cfg file. For additional
information on this feature, see:

https://wheel.readthedocs.io/en/stable/index.html#including-the-license-in-the-generated-wheel-file