[`pyflakes`] Treat function-scope bare annotations as locals per PEP 526 (`F821`) (#21540)

## Summary

Resolves #21494 

Treat function-scope annotated variables as locals per PEP 526 ,
preventing F821 false positives.

## Test Plan

Added tests for these cases in
`crates/ruff_linter/resources/test/fixtures/pyflakes/F821_34.py`