## Summary
Fixes#3011.
Type checkers currently allow forward references in all contexts in stub
files, and stubs frequently make use of this capability (although it
doesn't actually seem to be specc'd anywhere --neither in PEP 484, nor
https://typing.readthedocs.io/en/latest/source/stubs.html#id6, nor the
CPython typing docs). Implementing it so that Ruff allows forward
references in _all contexts_ in stub files seems non-trivial, however
(or at least, I couldn't figure out how to do it easily), so this PR
does not do that. Perhaps it _should_; if we think this apporach isn't
principled enough, I'm happy to close it and postpone changing anything
here.
However, this does reduce the number of F821 errors Ruff emits on
typeshed down from 76 to 2, which would mean that we could enable the
rule at typeshed. The remaining 2 F821 errors can be trivially fixed at
typeshed by moving definitions around; forward references in class bases
were really the only remaining places where there was a real _use case_
for forward references in stub files that Ruff wasn't yet allowing.
## Test plan
`cargo test`. I also ran this PR branch on typeshed to check to see if
there were any new false positives caused by the changes here; there
were none.
This PR fixes the following false positive in a `.pyi` stub file:
```py
x: int
y = x # F821 currently emitted here, but shouldn't be in a stub file
```
In a `.py` file, this is invalid regardless of whether `from __future__ import annotations` is enabled or not. In a `.pyi` stub file, however, it's always valid, as an annotation counts as a binding in a stub file even if no value is assigned to the variable.
I also added more test coverage for `.pyi` stub files in various edge cases where ruff's behaviour is currently correct, but where `.pyi` stub files do slightly different things to `.py` files.
Alex Waygood