Hello,

Pythons setuptools are deprecating the "python setup.py test" command:

https://github.com/pypa/setuptools/issues/1684

As you may know, "python setup.py test" is what python-build-system does
during the 'check' phase.

I'm not sure what we should do about it, and there does not seem to be
an established "community consensus" yet.  I suppose many packages will
be migrating to Pytest and/or Tox?  Perhaps we could check package
inputs and try to guess the correct test command?

Something to keep in mind for future python-build-system improvements...

PS: Apparently "python setup.py install" is going away too:
https://github.com/pypa/setuptools/issues/1684#issuecomment-508302910