Support `pyrefly`

[A5rocks]

pyrefly is one of the two new typecheckers. We should run it over our type tests to make sure our interface is supported, fixing anything necessary.

[A5rocks]

Blocked on facebook/pyrefly#43 because the self/cls parameter of decorated methods in trio.Path is generic (Self).

EDIT: nevermind I misunderstood, I made a new issue to track our problem! facebook/pyrefly#507