[py3] documentation generation is broken

[genodeftest]

Steps to Reproduce (for bugs)

1. Get the latest commits from py3 branch
2. run make check-doc or make -C doc html

Expected Behavior

Generate documentation or at least try to.

Current Behavior

Aborts with this error message:
Configuration error:
There is a programmable error in your configuration file:

Traceback (most recent call last):
  File "/usr/lib/python3.7/site-packages/sphinx/config.py", line 361, in eval_config_file
	execfile_(filename, namespace)
  File "/usr/lib/python3.7/site-packages/sphinx/util/pycompat.py", line 86, in execfile_
	exec(code, _globals)
  File "/path/to/exaile/doc/conf.py", line 46, in <module>
	mocks.fake_xl_settings()
  File "/path/to/exaile/doc/mocks.py", line 81, in fake_xl_settings
	import xl.settings
  File "/path/to/exaile/xl/settings.py", line 41, in <module>
	from xl.common import VersionError, glib_wait, glib_wait_seconds
  File "/path/to/exaile/xl/common.py", line 748, in <module>
	class ProgressThread(GObject.GObject, threading.Thread):
TypeError: __mro_entries__ must return a tuple

Possible Solution

I think someone needs to go over the config for sphinx and probably redo it.

Environment

Linux/Fedora with python3-3.7.6-1.fc31.x86_64 and python3-sphinx-2.1.2-2.fc31.noarch. I have also seen this with our ubuntu19.10 and debian10 docker images:
https://travis-ci.org/exaile/exaile/jobs/637118951
https://travis-ci.org/exaile/exaile/jobs/637118949

[rokm]

From what I can tell, this is caused by introduction of PEP 560 in python 3.7.

Our Mock implementation returns an instance of itself for __mro_entries__ (default behavior), causing the error. Raising AttributeError seems to cause another error later on, but adding a __mro_entries__ method that returns an empty tuple seems to do the trick.

However, as noted in the PR, I am not familiar enough with this area of python to be certain that there are not side effects.

[rokm]

Hmm, it seems the PR fixed the issue for pyhton 3.7.x but not for python 3.6.x - Ubuntu 18.04 is still failing, but with the metaclass conflict error that I've mentioned above.