- check build requirements for conflicts
- better isolation (ignore system site packages)
- support 2 prefixes: a "normal" one, and an "overlay" one
(with higher priority over "normal")
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.
