Stop building universal wheels

[schuderer]

References

As described in #291, Nox ist still building universal wheels and publishing them to PyPI with the py2.py3 identifier in the wheel's file name.

What was changed

• Remove setup.cfg outright (the bdist_wheel setting of universal = 1 was the only thing it contained)

Effects/tests

Tested as follows:

$ python setup.py sdist bdist_wheel  # wheel name is now marked as py3 instead of py2.py3
$ pip install dist/nox-2019.11.9-py3-none-any.whl
$ nox  # sucessfully runs it own tests
$ uninstall nox
$ pip install dist/nox-2019.11.9.tar.gz  # just to be safe
$ nox  # also still works, phew ;)

[dhermes]

LGTM