[IMPROV] Remove 'PY3' switch

Because Python 3 wasn't really supported until know, it was only
possible to run it with Python 3 if 'PY3' was set to '1'.

Change-Id: If0251bd5c9666e79a273c06b3ef4a3ea7995b0bf
Merge "[IMPROV] Remove 'PY3' switch"