Add --disallow-str-iteration flag

[michaelm-openai]

Fixes #11001

Related to typing python/typing#256

Expected behavior changes under this flag:

• str and its subtypes are no longer treated as subtypes of Collection, Iterable, Sequence or protocols that extend them. Assignment and argument passing will be treated as errors.
• The builtin iter() method has an additional overload which accepts str. This is the recommended way to iterate over characters when this flag is enabled.
• Invalid assignment of str to subtypes of Container, such as Collection will not report a conflict on the __contains__ method, since this introduces noise. It is known that str does not conform to the Container protocol due to its incompatible signature for __contains__ method.
• isinstance(foo, Sequence), etc. will narrow foo to str | Sequence[Any], etc. to be consistent with runtime behavior. That implies in order to iterate through foo, one must check isinstance(foo, Sequence) and not isinstance(foo, str).

[michaelm-openai]

I think the apprise diff is expected given that we added a new overload. The full diagnostic is:

apprise/config/base.py:1509: error: No overload variant of "iter" matches argument type "None"  [call-overload]
apprise/config/base.py:1509: note: Possible overload variants:
apprise/config/base.py:1509: note:     def [_SupportsNextT_co: SupportsNext[Any]] iter(SupportsIter[_SupportsNextT_co], /) -> _SupportsNextT_co
apprise/config/base.py:1509: note:     def iter(str, /) -> Iterator[str]
apprise/config/base.py:1509: note:     def [_T] iter(_GetItemIterable[_T], /) -> Iterator[_T]
apprise/config/base.py:1509: note:     def [_T] iter(Callable[[], _T | None], None, /) -> Iterator[_T]
apprise/config/base.py:1509: note:     def [_T] iter(Callable[[], _T], object, /) -> Iterator[_T]

[github-actions]

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

apprise (https://github.com/caronc/apprise)
+ apprise/config/base.py:1509: note:     def iter(str, /) -> Iterator[str]