Fixes type narrowing for overlaping runtime types

[sobolevn]

As always, fixing issues with type narrowing / inference is always very hard, because small changes can backfire.
But, this looks as a simple change. Let's see how tests will behave.

Closes #11272

[sobolevn]

pydantic/utils.py:554: error: has no attribute "class" [attr-defined]

I am not sure what to do with this one. Maybe I should create a PR for pydantic with proper assert_never helper here?

src/websockets/legacy/server.py:364: error: unused "type: ignore" comment

Looks like we fixed a typing bug here 🎉
Repro:

from typing import Optional, Callable, Awaitable

async def some(x: Optional[Callable[[], Awaitable[str]]]) -> str:
    if x is not None:
        response = x()
        if isinstance(response, Awaitable):
            reveal_type(response)  # N: Revealed type is "typing.Awaitable[builtins.str]"
            return await response
        else:
            reveal_type(response)  # N: Revealed type is "<nothing>"
            return response

pandas/core/indexers/utils.py:360: error: has no attribute "stop" [attr-defined]

Cannot reproduce this one for now 🤔

[github-actions]

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

pydantic (https://github.com/samuelcolvin/pydantic.git)
+ pydantic/utils.py:554: error: <nothing> has no attribute "__class__"  [attr-defined]

pandas (https://github.com/pandas-dev/pandas.git)
+ pandas/core/indexers/utils.py:360: error: <nothing> has no attribute "stop"  [attr-defined]
+ pandas/core/indexers/utils.py:360: error: <nothing> has no attribute "start"  [attr-defined]
+ pandas/core/indexers/utils.py:360: error: <nothing> has no attribute "step"  [attr-defined]
+ pandas/core/common.py:131: error: <nothing> has no attribute "dtype"  [attr-defined]

websockets (https://github.com/aaugustin/websockets.git)
+ src/websockets/legacy/server.py:364: error: unused "type: ignore" comment
+ src/websockets/legacy/server.py:592: error: unused "type: ignore" comment

[hauntsaninja]

Thanks for the fix!