There was an edge case where the user could pass something like
`venv_locaiton="~/tmp/venv"`, but this would be place in a directory
`$PWD/~/tmp/venv`.  So now use `os.path.expanduser(venv_location)` to
fix this.

Got rid of venv_location tests that created folder in home directory

The user can pass a string or Path object to set venv_location. Note
that for now, I left in `os.path.expanduser`. This coverts things like
`venv_location="~/path/to/dir"` to `$HOME/path/to/dir` instead of
`$PWD/~/path/to/dir`. If you really want the latter behavior, you can
pass in `Path.cwd().joinpath("~", "path", "to", "dir")`

* Now raises error if passing directly multiple pythons or parametrize
for session with `venv_location`
* With `venv_location` set, ignore (and warn) `--extra-python`
* Warn that session sets `venv_location`