python: Drop use of types.FunctionType.

python: Drop use of types.FunctionType.

This code asserted that the callback argument was of type
types.FunctionType.  It's more pythonic to just check that the argument
is callable, and not specifically that it's a function.  There are other
ways to implement a callback than types.FunctionType.

Signed-off-by: Russell Bryant <russell@ovn.org>
Acked-by: Ben Pfaff <blp@ovn.org>