Add a new testsuite option `--use-venv` to enable the use of `venv`
for creating a test virtual environment. The option is opt-in because
creating a `venv` environment does not work right when running under a
`virtualenv`; which is why `tox-venv` must be used in combination with
tox.
- cleanup virtualenv creation code
- ensure all testing virtual environments use a recent version
of setuptools / wheel, making it easier to switch to custom
versions of those, as well as reducing network accesses
- reduce size of testing virtual environment, slightly speeding
up the testsuite
importing it prevent to debug other packages with `-W error` as the
deprecation warning will raise.
Though there is still imp imported from a few vendorized packages,
and for other purposes than cache_from_source.
* 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
