Set[T] is incompatible with Set[Optional[T]] - Set[None]

[0az]

Tested with 0.761.

MWE:

from typing import Optional, Set

class Foo:
    pass

bar: Set[Optional[Foo]] = set()
baz: Set[Foo] = bar - {None}
# Incompatible types in assignment (expression has type "Set[Optional[Foo]]",
# variable has type "Set[Foo]")

[PeterJCLaw]

If support for this is added, maybe we could also consider including support for sets of Literal too?
(my use-cases is actually also the None case, but I'm guessing the same logic might work for both)

[PeterJCLaw]

Also possibly related - it would be great if mypy could understand the use of set.discard (etc.) to remove None elements.