Pure-interface

Fixes 91 Delegate subclasses no longer override base class methods and attributes.

This release removes abc.abstractmethod, abc.abstractclassmethod and abc.abstractstaticmethod from pure_interface.
Refactored python module into a package.
Added Delegate class.
Fixed bug handling generics 80.

Handle byte code generated with -OO flag in 3.10 and 3.11

Fixes bug not accepting empty async def methods.

Fixes issue 83 for interfaces with __call__ as one of the interface methods.

drops support for Python 2.7 and 3.5
Renames type_is_pure_interface to type_is_interface