fix false bad-return for a generator function with Iterator[X] | T return annotation #4825

[asukaminato0721]

Summary

Fixes #4825

Iterator and Iterable annotations now allow generator return values, while explicit Generator return types remain checked.

Test Plan

add test

[meta-codesync]

This pull request has been imported. If you are a Meta employee, you can view this in D118959974. (Because this pull request was imported automatically, there will not be any future comments.)

[github-actions]

Diff from mypy_primer, showing the effect of this PR on open source code:

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SUMMARY
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Total: +0 new errors, -2 fixed errors
By preset: +0/-1 (default), +0/-1 (strict)

Projects with changes (2):
  spark: +0 -1
  beartype: +0 -1
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FULL DIFF DETAILS
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spark (https://github.com/apache/spark)
- ERROR python/pyspark/sql/tests/arrow/test_arrow_udtf.py:1046:28-36: Returned type `Iterator[Unknown]` is not assignable to declared return type `None` [bad-return]

beartype (https://github.com/beartype/beartype)
- ERROR beartype/_util/func/utilfuncframe.py:1233:17-30: Returning implicit Any from function declared to return "None" [no-any-return-implicit]