The pip-specific Path implementation has been removed, and all its
usages replaced by pathlib.Path. The tmpdir and tmpdir_factory fixtures
are also removed, and all usages are replaced by tmp_path and
tmp_path_factory, which use pathlib.Path.
The pip() function now also accepts pathlib.Path so we don't need to put
str() everywhere. Path arguments are coerced with os.fspath() into str.
This essentially allows me to do an overall check general check by running the tests using pytest's `-k basic` syntax. Given that I like running tests often and that, in general, I make typos more often than changes that break core functionality, I think this will reduce cycle times for me.
