tests: use 'build' in tests instead of running setup.py

[henryiii]

Description

Use build instead of direct setup.py commands in our tests. Mostly by @abravalheri, pulled from #3711.

Suggested changelog entry:

* Avoid ``setup.py <command>`` usage in internal tests.

[rwgk]

Out of my wheelhouse but LGTM.
Would be nice to add a sentence or two to the PR description, for build muggles like myself:

• Was it the Python 2 or 3.5 drop that made this possible?
• Why are we keeping setup.py? (Needed externally?)

[henryiii]

setup.py <command> is very deprecated. We shouldn't be using or promoting it. Using build is the correct alternative. This still runs setuptools, which still uses setup.py internally, but the use of commands is an implementation detail. We still heavily (for now) use setup.py. That might change in the future, though. :)

No, don't think it was the Python drop, it was just... Not sure why I used raw setup.py commands before, probably didn't realize the proper way to do it. Though the drop does make it easier to just add a specific build version.