Support `NotRequired` on classes

To make an iterable that doesn't support `str`:
```py
class WorkProperlyIterable(Iterable[out_T], Protocol):
__contains__: NotRequired["(object) -> bool"]
```

The issue names no files or tests; start by locating the implementation of NotRequired handling and class-based Protocol member analysis. Confirm that the shown Python syntax is accepted for __contains__ and that the resulting iterable can omit str support without producing an invalid type-checking error.

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