Extension interferes with other autodoc-skip-member event handlers

[timobrembeck]

Any handler for the autodoc-skip-member signal should return None when the skipping behavior should not be changed:

If more than one enabled extension handles the autodoc-skip-member event, autodoc will use the first non-None value returned by a handler. Handlers should return None to fall back to the skipping behavior of autodoc and other enabled extensions.

At the moment, the value of skip is returned (which is True/False instead of None):

https://github.com/edoburu/sphinxcontrib-django/blob/471f23d3505f74bcff7bb1448781a85a3bc2bf8d/sphinxcontrib_django/docstrings/__init__.py#L147