Don't import anything from typing.py in our tests

[AlexWaygood]

There's a theoretical footgun here where we could accidentally be testing the typing.py implementation instead of our own implementation/re-export. It doesn't look like there's actually any issues here, but in any event, I think it's good for us to be explicit about when we actually want to use the typing.py version of a symbol by refering to it as typing.$symbol rather than just symbol