In a default setup, virtualenv-multiver no longer works correctly on

versions of Python prior to 3.8. The reason is that a modern

virtualenv will install the pip for the version of Python associated

with the virtualenv command, and that's too new these days for older

environments. So we need to explicitly download an appropriate version,

or not install one at all.
It's important that we give some control over this behavior, so we now

take arguments for controlling whether downloads occur and whether

pip, setuptools, and wheel get installed.
Along with this, we have options for --quiet/--verbose, --help,

and --version.
While here, some legacy unneeded code has been removed, to keep this a

bit more maintainable.

Made some virtualenvs using this, verifying it downloaded appropriate

versions of the packages for each version of Python.
Tested all the options to make sure they did what they were supposed

to.