Check attributes for variance violations

[JukkaL]

Similar to #734, but for attributes. In a covariant class an instance attribute with type List[T] would not be valid, since List is invariant.

[jhance]

I wonder if theres a way to word the error message here so it would be the same as #734?

[JukkaL]

Hmm.. can't come up with a good common message right now. Why not just come up with a separate message for attributes?

[JukkaL]

Also, assignment to a covariant attribute should be rejected. There doesn't seem to be issue for that.

[KotlinIsland]

Am I correct in thinking that an example of this issue is:

from typing import TypeVar, Generic
T = TypeVar("T", covariant=True)
class Foo(Generic[T]):
    foo: T
a: Foo[int] = Foo()
b: Foo[object] = a
b.foo = "sus"
a.foo + 1  # no mypy error, runtime error