Support using tox-venv for testing

The tox-venv plugin for tox leverages the venv module from the
Python 3 standard library instead of relying on the third-party
virtualenv project. Its environments differ in minor ways, however,
so to accommodate that we adjust a couple of our tox testenvs. The
dist testenv no longer assumes the wheel package will be installed,
and expressly includes it as a dep. The docs testenv preinstalls
mudpy into the environment so that we can be sure PBR generates the
AUTHORS and ChangeLog files before Sphinx attempts to include them
in any rendered documents.