Better `@Abstract` output for classmethods and staticmethods

[prusse-martin]

Abstract is designed to be used with instance methods.

class Foo:
    @Abstract
    def DoStuff(self):
        """Implementation to do stuff."""

Uses with classmethods are possible and work as expected

class Foo:
    @classmethod
    @Abstract
    def DoStuff(cls):
        """Implementation to do stuff."""

But when an implementation is missing in the sub class no reference to the offending class is present (just type).
In this issue we should add proper support for class methods (better error message) and better describe this in the docstring (also that staticmethods are not supported).