This adds support for config-specified Python versions in a .pydorc,

pyproject.toml, or setup.cfg when running virtualenv-multiver to

create an environment.
That helps a lot with consistency. Projects can now define the supported

versions, and virtualenv-multiver will then use those to create a

consistent environment.

Tested with specifying versions on the command line.
Tested with specifying versions in a config file.
Tested with no versions specified.