issubclass doesn't narrow down types

Bug Report

issubclass appears to not narrow down possible types.

To Reproduce

from typing import Any, Mapping, MutableMapping


def test_mutablemapping(obj: Mapping[Any, Any]) -> MutableMapping[Any, Any]:
    if issubclass(type(obj), MutableMapping):
        return obj  # error: Incompatible types in assignment (expression has type "Mapping[Any, Any]", variable has type "MutableMapping[Any, Any]")
    else:
        return dict(obj)

Expected Behavior

To narrow down in similar fashion to isinstance:

from typing import Any, Mapping, MutableMapping


def test_mutablemapping2(obj: Mapping[Any, Any]) -> MutableMapping[Any, Any]:
    if isinstance(obj, MutableMapping):
        return obj
    else:
        return dict(obj)

Your Environment

• Mypy version used: 0.910
• Mypy command-line flags: default
• Mypy configuration options from mypy.ini (and other config files): default
• Python version used: 3.8.8
• Operating system and version: Windows 10

Inizia con il riproduttore test_mutablemapping e confronta il suo ramo issubclass(type(obj), MutableMapping) con l'esempio isinstance nell'issue. Traccia la gestione da parte di mypy del restringimento dei tipi per issubclass, quindi aggiungi un test di regressione che mostri che il ramo restituisce MutableMapping[Any, Any] senza errori.