Infer yield-from item type for concrete Iterable subclasses

[apoorva-01]

Fixes #17449

yield from over an Iterator/Iterable subclass was inferring Any, so this slipped by:

def f() -> Iterator[str]:
    yield from Ints()  # Ints is Iterator[int], silently ok

get_generator_yield_type only handles the generator types themselves, and a concrete subclass isn't one, so it bailed to Any. Now it maps to Iterable and takes the item type, like a for loop.

Also dodges the args[0] trap #20983 hit (enumerate came out str instead of tuple[int, str]). tests cover both, both fail without the fix.

[github-actions]

According to mypy_primer, this change doesn't affect type check results on a corpus of open source code. ✅