WIP: [python-package] [ci] move some configuration to setup.cfg

[jameslamb]

Contributes to #5061 .
Reduces the number of changes that need to be reviewed when switching build backends for the Python package (work-in-progress in #5759).

This PR proposes moving some configuration for the Python package out of setup.py and into a setup.cfg file. This moves the project one step closer to having PEP 517 / 518 compliant Python package builds.

For reference, see:

• "Configuring setuptools using setup.cfg files" (setuptools docs)
• PEP 517 (link, accepted in September 2017)

[jameslamb]

blegh I guess I misunderstood, and you can't mix some arguments supplied via setup.py and others via setup.cfg the way I was trying here. I'll close this, sorry.