Document that `inspect.isclass` returns False for `GenericAlias` instances

Bug report

Bug description:

Hi
I am not sure if this is a bug, but I use the bug template because I couldn't find a better one to submit questions like this one.

The documentation says, "User-defined generic classes can be instantiated.".

import inspect
from typing import TypeVar, Generic

T = TypeVar("T")


class Node(Generic[T]):
    x: T  # Instance attribute (see below)

    def __init__(self, label: T | None = None) -> None:
        self.label = label


x = Node("")  # Inferred type is Node[str]
y = Node(0)  # Inferred type is Node[int]
z = Node()  # Inferred type is Node[Any]


# Also OK:
y = Node[int](0)


print(inspect.isclass(Node))  # True
print(inspect.isclass(Node[int]))  # False  (shouldn't this be True?)
print(inspect.isclass(Node[str]))  # False  (shouldn't this be True?)

In the example above, shouldn't calls such as inspect.isclass(Node[int]) return True, since they can be instantiated and used like normal classes?

CPython versions tested on:

3.13

Operating systems tested on:

Linux

Linked PRs

• gh-133504
• gh-150854
• gh-151116
• gh-151117