Special-casing `__hash__`

The concept of "hashable" and "unhashable" classes has been a bit of the sore point in the past. typeshed uses __hash__: ClassVar[None] to mark classes as non-hashable, although that requires a # type: ignore[assignment] annotation. I think this is special-cased by at least some type checkers for marking classes as unhashable.

Except for some wordage in the dataclasses section, the typing spec is silent about hashability. Since I believe that this needs special-casing by type checkers, it should be added to the typing spec. A raw idea:

• Classes are hashable by default (due to object.__hash__() being implemented and typeshed having a type corresponding type annotation.)
• To mark a class as non-hashable, use __hash__: ClassVar[None]. We could alternatively use something simpler such as __hash__ = None. I believe we used to use that, but I don't know why it's changed.
• To mark a class as hashable, use def __hash__(self, ...) -> ..., usually def __hash__(self) -> int: ....
• Sub-classes inherit their parent's hashability, unless overridden.

Start by reading the typing spec, especially the existing dataclasses section, and review the discussion and proposed alternatives in this issue. Define the agreed rules for default hashability, __hash__: ClassVar[None] or __hash__ = None, explicit __hash__ methods, and inheritance; done means the typing spec documents the accepted behavior.