Stop supporting isinstance().

The big exception is isinstance(f, Callable), which is now equivalent
to isinstance(f, collections.abc.Callable).  Also other enhanced ABCs
that shadow ABCs from collections.abc still support isinstance() --
but not the concrete classes (List, Dict, Set, FrozenSet).

I'm sad to see the original Callable.__instancecheck__ go, but Mark
Shannon's right that isinstance() with a type argument is just poorly
defined.

Fixes #135.

Author: Guido van Rossum