Are all the -> None necessary?

Nope, type annotations are completely optional in Python :)

More seriously, they could be omitted, as we're not running mypy on this repo (yet).

And generally, it is a good idea to also type hints to tests (for example, see https://sethmlarson.dev/tests-arent-enough-case-study-after-adding-types-to-urllib3#type-your-tests) and that could cover these returns too.

It can be useful in tests to help immediately differentiate from fixtures and helper methods which do return something, for example in #288.

I was referring specifically to the tests (not fixtures or other helper functions), since they (always?) return None, making the -> None superfluous.
I guess it makes sense to add it for consistency, both with other functions/methods that have the return type, or for tests that have types for the their args?

Yeah, we can remove them if you prefer, I don't mind too much either way. But when we add mypy, let's add them back if it complains about it.

Adding them opt-in the function to be checked by mypy, so it will be usefull if we use mypy on this repo sometime.