docs: Improve the Mock class

The python import machinery expects __path__ to be a string, so
__getattr__ has to return a mock string instead of a mock object.

__getattr__ now also returns Mock objects instead of the Mock class
and Mock objects are now callable.

docs/faq.rst

@@ -38,9 +38,13 @@ You can mock out the imports for these modules in your conf.py with the followin
         def __init__(self, *args, **kwargs):
             pass
 
-        def __getattr__(self, name):
+        def __call__(self, *args, **kwargs):
             return Mock()
 
+        @classmethod
+        def __getattr__(self, name):
+            return Mock() if name not in ('__file__', '__path__') else '/dev/null'
+
     MOCK_MODULES = ['pygtk', 'gtk', 'gobject', 'argparse']
     for mod_name in MOCK_MODULES:
         sys.modules[mod_name] = Mock()