Assert statements should analyze isinstance

[jhance]

I can imagine this being useful for testing (where asserts are actually common and useful). Something like this should typecheck:

class A:
    pass

class B(A):
    attr = None

x = B() # type: A
assert isinstance(x, B)
x.attr

[jhance]

oops was a dup

[o11c]

For reference, the duplicate is #475