py3: fix documentation generation on python 3.6+ (try 2)

[rokm]

Second attempt at fixing the documentation generation issue, tested both with python 3.6.x on Ubuntu 18.04 and 3.7.x on Fedora 31.

This time, the __mro_entries__ issue is addressed by raising the AttributeError.

However, this still leaves the TypeError: metaclass conflict: the metaclass of a derived class must be a (non-strict) subclass of the metaclasses of all its bases error, which is caused by the mocked GObject.GObject being involved in multiple inheritance in xl.common.ProgressThread. For this to work, we need our Mock object for the GObject module return a type instead of an instance for GObject (class).

Due to the name repetition (gi.repository.GObject.GObject, with GObject referring both to the module and the class) and __getattr__ being a class method, distinction between the two is handled by introduction of a new MockGiModule class, which now also collects all mock functionality of GI (sub)modules.

[genodeftest]

Documentation builds and works fine and nothing breaks so feel free to merge imho. Thanks for caring!