Fix overload overlap check for UninhabitedType

[ilevkivskyi]

The issue was exposed by merge of subtype visitors. Fix is actually trivial, but the diff is big because I need to add and pass the new flag everywhere (is_subtype(), is_proper_subtype(), is_equivalent(), is_same_type() can call each other).

cc @Michael0x2a

[ilevkivskyi]

mypy_primer output is technically correct. We (intentionally?) use is_subtype_no_promote() everywhere for overload checks. And this PR passes subtype flags to nested subtype checks. So people need to write e.g. Union[Iterable[int], Iterable[float]] in the implementation signature.

I am not sure why do we require this? If it looks to strict, I think it is easy to go back to using just is_subtype(). @Michael0x2a what was the motivation to ignore promotions during overload implementation signature check?

[ilevkivskyi]

Well, FWIW I switched to is_subtype() and all tests passed. So I guess it was not really an intentional decision. Let's see what it will do here.

[github-actions]

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

pandas (https://github.com/pandas-dev/pandas)
- pandas/core/apply.py:510: error: Key expression in dictionary comprehension has incompatible type "Hashable"; expected type "NDFrame"  [misc]

[ilevkivskyi]

Yeah, mypy_primer looks better now.

[hauntsaninja]

Looks great!