python3: Fix pyc-only related runtime exceptions

Python3 changes the pyc lookup strategy, ignoring the
__pycache__ directory if the .py file is missing. Change
install location to enable use of .pyc without their parent .py

See http://www.python.org/dev/peps/pep-3147



[Peter: add patch header]
Signed-off-by: Daniel Nelson <daniel@sigpwr.com>
Acked-by: Arnout Vandecappelle (Essensium/Mind) <arnout at mind.be>
  (untested)
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>