Add some missing generic type arguments

[RobertCraigie]

This PR adds generic type arguments to type hints in sessions.py, I did not have time to go through the whole codebase unfortunately.

The benefit of adding type arguments is that type checkers like Pyright will complain if you access a variable that has a potentially unknown type, for example I ran into this:

import nox

@nox.session(reuse_venv=True)
def pyright(session: nox.Session) -> None:
    session.env['ENV_VARIABLE'] = '1'
    # Error: Type of "env" is partially unknown
    #            Type of "env" is "dict[Unknown, Unknown]"

If you want to avoid errors like these in the future, I'd highly recommend adding pyright to CI and using --verifytypes.

[henryiii]

I'd recommend turning up the strictness of mypy, and then this would be enforced. I can take a stab at it.

[FollowTheProcess]

Hi @RobertCraigie thanks for the PR! This looks great to me

[FollowTheProcess]

I'm gonna get this one in so @RobertCraigie is recognised as a contributor then planning to merge #596 straight after to enforce going forward, unless anyone from the maintainer team screams "No!" in the meantime 👍🏻