Basic python3 testing packages

Includes:
* user/py3-tox
  * user/py3-setuptools
  * user/py3-pluggy
  * user/py3-virtualenv
  * user/py3-pytest
    * user/py3-py

Most of these packages use tox itself for tests, so !check until we have
a copy of tox on the mirrors with which to bootstrap.