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.
`ops` is not being extended--if I recall correctly, those deletion operations are being inserted to specific locations in the list (which is why they must be reverse-sorted first). If these are not critical changes, then I'd prefer to just leave it the way it was. The flowcharts are a pain to test and debug (because I still have no unit tests for them).