Put back undoc-members option in sphinx test (#4586)

The test will fail without this due to the bug explained in #4584.

t/unit/contrib/proj/conf.py

@@ -4,7 +4,7 @@ import os
 import sys
 
 extensions = ['celery.contrib.sphinx', 'sphinx.ext.autodoc']
-autodoc_default_flags = ['members']
+autodoc_default_flags = ['members', 'undoc-members']
 autosummary_generate = True
 
 sys.path.insert(0, os.path.abspath('.'))