New review comment by ahesford on void-packages repository

https://github.com/void-linux/void-packages/pull/26883#discussion_r534413104

Comment:
Not a bad idea, but I've gone one step further. Running `python setup.py test` complains about deprectation and suggests `tox` instead. Because that is the official recommendation, I try `tox` first if possible, otherwise try `pytest` if possible, and finally fall back to the old behavior for non-PEP517 builds.

I've modified a few templates as an example of how the change eliminates some redundancy.

If there is not significant debate about the PEP517 changes, I'll squash the two `build-style/python3-module` commits before merging. For now, I'll keep them apart so we can separate the two issues if need be.