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.
Use pyupgrade to convert simple string formatting to use f-string
syntax. pyupgrade is intentionally timid and will not create an f-string
if it would make the expression longer or if the substitution parameters
are anything but simple names or dotted names.
using object that have the same name as submodules as the weird effect
of makeing `import pip.commands.<something> as <anothername>` fail with
a key error. This fixes it by renamin commands as command_dict and fixin
a few imports to accomodate.
Related to #2149
* Move virtualenv creation out of TestPipEnvironment
* Remove global state and force explicit use of TestPipEnvironment
instances
* Remove "backup" virtualenv copying and instead create new
virtual environments each time.
* Remove the monkeypatched "PyPICache" functionality
* Remove things that were not being used anymore and were dead
weight
* Remove sitecustomize support which was primarily used to
monkeypatch the "PyPICache" but was used in one or two other
tests.
* Better output with the bare asserts we use throughout the tests
* Function fixtures are pretty nice, especially as a way to
start a background server or create an isolated virtualenv