Keep Any when restricting Union in isinstance

[ilevkivskyi]

Fixes #1720

Currently, mypy incorrectly determines restrictions of unions containing Any after isinstance(), for example:

def func(v: Union[int, Any]) -> None:
    if isinstance(v, int):
        reveal_type(s) # This is correctly int
    else:
        reveal_type(s) # But this is incorrectly Union[<ERROR>, void], should be Any

This PR fixes this by not considering Any a subtype-of-every-type when restricting the union.

(Note that this PR is not a replacement for #2197 but a complement to it.)

[gvanrossum]

Probably best for @ddfisher or @JukkaL to review.

[JukkaL]

Looks good -- thanks!