The main setuptools PEP 517 backend is intended for
explicit usage in `pyproject.toml`, when the project
authors can ensure that their `setup.py` runs without
that directory being implicitly on `sys.path`.
For implicit usage, setuptools now offers a separate
legacy backend that more closely mimics direct
execution of the `setup.py` script.
- 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.
- 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
