Why are the type parameters `set`, `frozenset`, `dict` and `frozendict` are generic in unbounded?

[jonathandung]

Excuse me if this question may come across as stupid, but these hashed containers should only have hashable items. However, in builtins.pyi, I see that their type variables do not have an upper bound of collections.abc.Hashable. How is that so?

[JelleZijlstra]

Historically Hashable has been a bit of a problematic type, because hashability is sometimes hard to model (e.g., object is hashable but its subclasses may not be; there is __hash__ = None). Worth trying though and seeing what mypy-primer has to say.

[jonathandung]

Should I open a PR?