bpo-27984: explictly check type of singledispatch.dispatch() argument

[zhangyangyu]

https://bugs.python.org/issue27984

[zhangyangyu]

backport to 3.6: #6115

[ambv]

As commented on the BPO issue: the problem described in this issue is fixed, by BPO-32227.

The wrapper returned by the @register decorator is calling dispatch() with the first argument's __class__. It can only ever be invalid if somebody deliberately wrote something invalid to the object's __class__. It's extremely unlikely.

We should not slow down calling of all generic functions on the basis that somebody might pass a non-type straight to dispatch().