Python3 related fixes

Two fixes related to Python3:
- py.test for python3 is not necessarily called py.test3, and
- do not inject python2 builtins in the python3 builtins namespace.
Actually I think the idiomatic way to write this is `dels.append((lastInd+1, ('d', t)))` followed by `ops.extend(sorted(dels, reverse=True))` but I don't feel strongly about it either.