Add FrozenSet to the typing stub files

[Vgr255]

Followup from python/typing#261

Now, mypy sees that FrozenSet is in the module, but it complains that it's an invalid type. I'm probably overlooking something simple, I guess.

[gvanrossum]

Oh, TypeAlias() only works for types that have hard-coded support in mypy. But I think you can make it derive from AbstractSet and you probably won't have to add anything else.

[Vgr255]

Aah, fair enough. I still have a bit to learn about mypy.

[Vgr255]

Force-pushed new commit; mypy effectively detects it. Cheers!

[gvanrossum]

Thanks!