py: Implement __getattr__.

It's not completely satisfactory, because a failed call to __getattr__
should not raise an exception.

__setattr__ could be implemented, but it would slow down all stores to a
user created object.  Need to implement some caching system.

py/qstrdefs.h

@@ -26,6 +26,7 @@ Q(__add__)
 Q(__sub__)
 Q(__repr__)
 Q(__str__)
+Q(__getattr__)
 
 Q(micropython)
 Q(byte_code)