* 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

* 📰

This saves a network hop when using git and passing an explicit sha
as a ref by comparing the version that's already checked out.

Yields a ~4x speedup on my local machine

Before:
```
$ /usr/local/bin/pip --version
pip 7.1.0 from /usr/local/lib/python2.7/site-packages (python 2.7)
$ time /usr/local/bin/pip install --disable-pip-version-check -e git+https://github.com/getsentry/raven-python.git@56fc6f7beecf445843d0ec7052bb8c6f0ea80a2e#egg=raven_dev
Obtaining raven-dev from git+https://github.com/getsentry/raven-python.git@56fc6f7beecf445843d0ec7052bb8c6f0ea80a2e#egg=raven_dev
  Updating ./src/raven-dev clone (to 56fc6f7beecf445843d0ec7052bb8c6f0ea80a2e)
  Could not find a tag or branch '56fc6f7beecf445843d0ec7052bb8c6f0ea80a2e', assuming commit.
Installing collected packages: raven-dev
  Running setup.py develop for raven-dev
Successfully installed raven-dev
/usr/local/bin/pip install --disable-pip-version-check -e   0.84s user 0.48s system 39% cpu 3.300 total
```

After:
```
$ /Users/matt/.virtualenvs/pip/bin/pip --version
pip 7.2.0.dev0 from /Users/matt/code/pip (python 2.7)
$ time /Users/matt/.virtualenvs/pip/bin/pip install --disable-pip-version-check -e git+https://github.com/getsentry/raven-python.git@56fc6f7beecf445843d0ec7052bb8c6f0ea80a2e#egg=raven_dev
Obtaining raven-dev from git+https://github.com/getsentry/raven-python.git@56fc6f7beecf445843d0ec7052bb8c6f0ea80a2e#egg=raven_dev
checking version
  Skipping because already up-to-date.
Installing collected packages: raven-dev
  Running setup.py develop for raven-dev
Successfully installed raven-dev
/Users/matt/.virtualenvs/pip/bin/pip install --disable-pip-version-check -e   0.59s user 0.22s system 98% cpu 0.824 total
```

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.

* 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

Based on original patch by Anrs Hu.

Blocking release, the tests are very path orientated this needs fixup so
added TODO and skip for now.

remove "git submodule foreach" call for backwards compatibility; expect errors in pip install because old versions of git write senseless things to stderr