This test has been replaced by test_freeze_editable_git_with_no_remote()
in test_freeze.py, with an updated and different test expectation.

This addresses a review comment by @pradyunsg.

* Add failing test

* Don't assume master branch is default

* Don't expect output to stderr

* Use the 'short ref' instead of the 'full ref'

* Run isort on the codebase

Command: isort --recursive --skip __main__.py --skip _vendor -o pip._vendor -sd THIRDPARTY -m 5 -p pip -p tests ./pip ./tests

* 📰

flake8 ignores some errors by default, these ignored by defaults
change sometimes. This will make things stabler by selecting
everything.

* 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.