Oop-ext

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* Interface and implementation methods can no longer contain mutable defaults, as this is considered
a bad practice in general.

* ``Null`` instances are now hashable.

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* Fix mismatching signatures when creating "interface stubs" for instances:

.. code-block:: python

foo = IFoo(Foo())

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* Interfaces now support keyword-only arguments.

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* Remove ``FunctionNotRegisteredError`` exception, which has not been in use for a few years.

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* Fix issues of ignored exception on nested callback.

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* Fix issues and remove obsolete code.