Let --allow-redefinition-new widen global in function with None init

[JukkaL]

Now this infers an optional type, for backward compat with --allow-redefinition-old:

# mypy: allow-redefinition-new
x = None

def foo() -> None:
    global x
    x = 1  # Ok
    
# Type of "x" is "int | None" here

There are a few non-trivial things. The implementation propagates the widened type to the global scope, since otherwise x would have type None at top level after the function. We also only allow widening if the original type is None, to make reasoning about this easier. Otherwise the type could be inferred from any number of functions. Also this is sufficient to maintain backward compatibility.

I used coding agent assist with small incremental changes and careful manual reviews.

See #21276 for additional context.

[github-actions]

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

[ilevkivskyi]

Yeah, this is a bit ugly, but not too bad to maintain backwards compatibility for an important edge case.

I have just one suggestion.

[ilevkivskyi]

Maybe give some example relevant test cases here? Like e.g. testNewRedefineGlobalVariableNoneInit tests