fix: unpack unions inside tuples in except handlers

[subnix]

Fixes #17759

mypy unpacks only top-level unions in except handlers, leaving unions inside tuples unchanged. This leads to the failed check because the Union type isn't a subtype of the BaseException type. We can fix this by simplifying and unpacking types inside tuples.

Before the patch:

source = Tuple[Union[Type[ValueError], Type[RuntimeError]], Type[ZeroDivisionError]]
result = [Union[Type[ValueError], Type[RuntimeError]], Type[ZeroDivisionError]]

After the patch:

source = Tuple[Union[Type[ValueError], Type[RuntimeError]], Type[ZeroDivisionError]]
result = [Type[ValueError], Type[RuntimeError], Type[ZeroDivisionError]]

[subnix]

@JelleZijlstra since you initially made the fix (#2846), could you please review it?

[github-actions]

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

mongo-python-driver (https://github.com/mongodb/mongo-python-driver)
+ pymongo/socket_checker.py:82: error: Unused "type: ignore" comment  [unused-ignore]

[hauntsaninja]

Thank you!