Improve/clean up packaging

[jwodder]

See each commit message for explanations of the changes.

[coveralls]

Coverage decreased (-0.4%) to 86.156% when pulling 523f547 on jwodder:packaging into 076c56f on duecredit:master.

[yarikoptic]

I will drop python2 and resolve conflicts (via merge)

[jwodder]

@yarikoptic If you're dropping Python 2 support, you should also get rid of the [bdist_wheel]universal setting in setup.cfg and set python_requires in setup.py.

[yarikoptic]

thanks, done. Hopefully did it right ;-)

[yarikoptic]

ok, CI is green. will not dive into coverage difference at this point. Thank you @jwodder -- next step is a release to prepare for #178