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.

Beginne damit, die Typing-Spezifikation zu lesen, insbesondere den bestehenden Abschnitt zu Dataclasses, und sieh dir die Diskussion und die vorgeschlagenen Alternativen in diesem Issue an. Definiere die vereinbarten Regeln für die standardmäßige Hashbarkeit, __hash__: ClassVar[None] oder __hash__ = None, explizite __hash__-Methoden und Vererbung; abgeschlossen bedeutet, dass die Typing-Spezifikation das akzeptierte Verhalten dokumentiert.