The with_wheel fixture doesn't have a value and isn't ever referenced
within a test function.

To avoid typing it every test function, change tests to use
pytest.mark.usefixture. It will no longer appear as an argument.

https://docs.pytest.org/en/latest/how-to/fixtures.html#use-fixtures-in-classes-and-modules-with-usefixtures

Make more tests run with with_wheel and test
that a .dist-info directory is created.

This is the default, so there is no need to specify it explicitly.

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.

* 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.