Tag safir.asyncio docs with :inherited-members:

sphinx_automodsumm's ability to discover the methods of a class
appears to have some bug with some but not all classes that inherit
from Generic that seems to be related to the use of __slots__ to
determine the methods of the class. Tagging safir.asyncio with
:inherited-members: changes the code to use dir() to retrieve all
members instead, which avoids the problem.

docs/api.rst

@@ -10,6 +10,7 @@ API reference
 
 .. automodapi:: safir.asyncio
    :include-all-objects:
+   :inherited-members:
 
 .. automodapi:: safir.click
    :include-all-objects: