Fix isinstance check for Generic classes

[dolfinus]

After upgrading to typing-extensions==4.6.1 this code example started failing on Python 3.7:

from typing_extensions import Protocol, runtime_checkable
from typing import Generic, TypeVar
from pathlib import Path
from dataclasses import dataclass

@runtime_checkable
class ContainsException(Protocol):
    """
    Protocol for objects containing ``.exception`` attribute
    """

    @property
    def exception(self) -> Exception:
        """
        Exception object with traceback
        """

T = TypeVar("T")

@dataclass(eq=False, frozen=True)
class PathContainer(Generic[T]):
    path: T
    exception: Exception

@dataclass(eq=False, frozen=True)
class PathWithFailure(PathContainer[Path]):
    pass


file = PathWithFailure(path=Path('/some'), exception=RuntimeError("some"))
isinstance(file, ContainsException)

Traceback (most recent call last):
  File "/home/maxim/Repo/typing_extensions/1.py", line 33, in <module>
    isinstance(file, ContainsException)
  File "/home/maxim/Repo/typing_extensions/src/typing_extensions.py", line 599, in __instancecheck__
    if super().__instancecheck__(instance):
  File "/home/maxim/.pyenv/versions/3.7.8/lib/python3.7/abc.py", line 139, in __instancecheck__
    return _abc_instancecheck(cls, instance)
  File "/home/maxim/Repo/typing_extensions/src/typing_extensions.py", line 583, in __subclasscheck__
    return super().__subclasscheck__(other)
  File "/home/maxim/.pyenv/versions/3.7.8/lib/python3.7/abc.py", line 143, in __subclasscheck__
    return _abc_subclasscheck(cls, subclass)
  File "/home/maxim/Repo/typing_extensions/src/typing_extensions.py", line 661, in _proto_hook
    and other._is_protocol
AttributeError: type object 'PathWithFailure' has no attribute '_is_protocol'

Who said that objects of generic class must have _is_protocol property?

Caused by #184

[cpython-cla-bot]

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[AlexWaygood]

Thank you! I think this probably also needs a changelog entry (under a new "Unreleased" heading at the top of the file)

[AlexWaygood]

(A test would also be great, to ensure that this doesn't regress again)

[dolfinus]

Added test and changelog item

[AlexWaygood]

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[AlexWaygood]

LGTM. I confirmed that the new test fails on main, and passes with this PR. Sorry again for the breakage!

[JelleZijlstra]

This fix was released as part of version 4.6.2 just now.