Expose test dependencies outside of noxfile.py

[brettcannon]

Right now the only way to run the tests outside of running nox -s tests is to manually install what's hard-coded there:

packaging/noxfile.py

Line 31 in 44efc1f

session.install("coverage[toml]>=5.0.0", "pretend", "pytest>=6.2.0", "pip>=9.0.2")

This is problematic if you want to set up your editor for running your tests before you take the next step with nox to run the tests on all installed versions of Python.

We could create a test optional dependency so that we can change:

packaging/noxfile.py

Line 32 in 44efc1f

session.install(".")

to be .[test]. And I'm specifically suggesting test over tests because that's what the core metadata chose as a name.

We could also do this for documentation with a doc extra.

[pradyunsg]

I won't mind mirroring pip -- and having a docs/requirements.txt and tests/requirements.txt. :)

[brettcannon]

That also works for me if that's the route you want to go.