test: Call Message.[i]recv() without arguments
Fix truth value testing for Exception instances in Python 2
test: More tests for error codes and Exception class
test: Add tests for Request.[wait|test]any()
test: Add tests for Op.Is_commutative()
test: Add test for Group.Dup()
test: Add more tests for Datatype
test: More tests for matched probes
pickle: Accept a user's dumps() function with a single argument