Narrow down tuple types with len check

[ikamensh]

Feature
I wish mypy understood that following code is actually correct:

def foo(x: tuple[str, str] | tuple[str, str, str]):
  if len(x) == 2:
     obs, info = x  # error: Too many values to unpack (2 expected, 3 provided)  [misc]
  ...

Pitch

Tuples are universal way to return multiple values in python, and are ubiquitous. It's natural to process cases where data can come in different number of elements with length checks. Currently I could write a custom type guard function (is boilerplate and extra effort) or # type: ignore (but I lose benefit of type checking here)

pyright understands this case, and I wish mypy did as well.

[AlexWaygood]

Duplicate of #1178