* 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.
* PEP381 Mirroring support was never fully implemented leaving
users of it trivially exploitable to a MITM or malicious mirror
operator.
* 2 out of 6 of the mirrors have been removed from the pool and
will never resolve.
* The remaining mirrors often fall behind
* The mirrors will likely never be available under HTTPS
* People who wish to use a mirror of PyPI can still do so by
manually specifying a mirror url for --index-url or
--extra-index-url which is more flexible, allowing for
mirrors to be hosted under any domain.