Support extras_require dependencies

[p7g]

I'm using riot for a package that has a couple of "contrib" sub-packages that add some support for 3rd party libraries. To manage the peer dependencies for these, I'm using the extras_require dict in setup.py.

I have a Venv to run tests, but for those tests I need to have the extra requirements installed, I guess using something like this pip install -e .[feature1].

It would be cool if this was possible, maybe something like this:

venv = Venv(
	name="test"
	pys=3.8,
	extras=["feature1", "feature2"],
	pkgs={...},
)

In the meantime, I think I'll work around this by just adding the dependencies to the pkgs of the test Venv.