Docs: Fix creating of docs of compoundselection.py

[pythonic64]

It's a possible fix for readthedocs error while creating docs.

Traceback from readthedocs:

Configuration error:
There is a programable error in your configuration file:

Traceback (most recent call last):
  File "/home/docs/checkouts/readthedocs.org/user_builds/kivy/envs/7387/lib/python3.7/site-packages/sphinx/config.py", line 161, in __init__
    execfile_(filename, config)
  File "/home/docs/checkouts/readthedocs.org/user_builds/kivy/envs/7387/lib/python3.7/site-packages/sphinx/util/pycompat.py", line 150, in execfile_
    exec_(code, _globals)
  File "conf.py", line 73, in <module>
    import autobuild
  File "/home/docs/checkouts/readthedocs.org/user_builds/kivy/checkouts/7387/doc/autobuild.py", line 35, in <module>
    import kivy.core.gl
  File "/home/docs/checkouts/readthedocs.org/user_builds/kivy/envs/7387/lib/python3.7/site-packages/kivy/core/gl/__init__.py", line 83, in <module>
    import kivy.core.window  # NOQA
  File "/home/docs/checkouts/readthedocs.org/user_builds/kivy/envs/7387/lib/python3.7/site-packages/kivy/core/window/__init__.py", line 28, in <module>
    from kivy.uix.behaviors import FocusBehavior
  File "/home/docs/checkouts/readthedocs.org/user_builds/kivy/envs/7387/lib/python3.7/site-packages/kivy/uix/behaviors/__init__.py", line 90, in <module>
    from kivy.uix.behaviors.compoundselection import CompoundSelectionBehavior
  File "/home/docs/checkouts/readthedocs.org/user_builds/kivy/envs/7387/lib/python3.7/site-packages/kivy/uix/behaviors/compoundselection.py", line 137, in <module>
    _is_desktop = Config.getboolean('kivy', 'desktop')
AttributeError: 'NoneType' object has no attribute 'getboolean'

/home/docs/checkouts/readthedocs.org/user_builds/kivy/envs/7387/lib/python3.7/site-packages/kivy/__init__.py

Maintainer merge checklist:

• Title is descriptive/clear for inclusion in release notes.
• Applied a Component: xxx label.
• Applied the api-deprecation or api-break label.
• Applied the release-highlight label to be highlighted in release notes.
• Added to the milestone version it was merged into.
• Unittests are included in PR.
• Properly documented, including versionadded, versionchanged as needed.

[matham]

It certainly can't hurt.