Update doc/conf.py to avoid warnings with sphinx 1.8

The autodoc_default_flags option is now deprecated.[1] This warning can
cause documentation build failed. use autodoc_default_options instead.

[1]https://github.com/matplotlib/matplotlib/pull/11928
Change-Id: I73041da072edb635661503b75a9c7460e1242220

doc/source/conf.py

@@ -274,4 +274,4 @@ extlinks = {
     'issue': ('https://github.com/dreamhost/stevedore/issues/%s', 'issue '),
 }
 
-autodoc_default_flags = ['members', 'special-members', 'show-inheritance']
+autodoc_default_options = {'members': None, 'special-members': None, 'show-inheritance': None}