Test for "if not PY2" in compatibility code, rather than "if PY3"

[plamut]

At a few place the Python 2/3 compatibility code tests if Python 3 is used, assuming the "else" scenario is Python 2. Such tests will break if Python 4 (or some other post-3 version) is ever released.

The fix is to instead split the code paths into "Python 2" and "not Python 2" groups.