Use dependency groups for test requirements

[notatallshaw]

This moves the test requirements out of requirements files into the pyproject.toml as dependency groups.

Some choices I made:

1, I removed instructions about building a local setuptools wheel, as I don't think that's too relevant any more?
2. I just used the dependency group names directly in the nox script, I originally had a dependency group dictionary but it just mapped "test" to "test" and "test-common-wheels" to "test-common-wheels"
3. I have not created a "docs" dependency group as the requirements refers to a local directory, and groups can not include this so the instructions to install docs requirements would include both a group and a local file

Apart from being nice that this is using a feature that was worked on for pip in the way it was intended, the main advantage I think is it allows other tooling and users to more easily find these additional dependencies. I think it's more common to skim over a pypyoject.toml now than it is to dig around looking for requirements.txt files.

[ichard26]

Glad to dogfood more of our own modern features 👍