[stdlib][types] fix *_frame type (may be None)

[x42005e1f]

This is a simple PR that adds None as a possible type for GeneratorType.gi_frame, CoroutineType.cr_frame, and AsyncGeneratorType.ag_frame: the property value becomes None when the object completes (e.g. closes). It is valid for all supported versions of Python (and even older ones).

Closes #15218.

[github-actions]

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

trio (https://github.com/python-trio/trio)
+ src/trio/_util.py:301: error: Item "None" of "FrameType | None" has no attribute "f_globals"  [union-attr]
+ src/trio/_core/_tests/test_asyncgen.py:309: error: Item "None" of "FrameType | None" has no attribute "f_locals"  [union-attr]
+ src/trio/_core/_tests/test_asyncgen.py:313: error: Item "None" of "FrameType | None" has no attribute "f_locals"  [union-attr]

[x42005e1f]

The claim that it is valid for all supported versions of Python can also be verified via the inspect.get*state() source code:

# Python ≥3.2 (but the behavior is present on ≥2.5 — PEP 342)
# https://github.com/python/cpython/issues/42099#issuecomment-1094010374
def getgeneratorstate(generator):
    ...
    if generator.gi_frame is None:
        return GEN_CLOSED
    ...

# Python ≥3.5 (PEP 492)
def getcoroutinestate(coroutine):
    ...
    if coroutine.cr_frame is None:
        return CORO_CLOSED
    ...

# Python ≥3.12 (but the behavior is present on ≥3.6 — PEP 525)
def getasyncgenstate(agen):
    ...
    if agen.ag_frame is None:
        return AGEN_CLOSED
    ...

[srittau]

Thanks!