hpfax tries to write bytecode cache in CUPS backend directory

The hpfax backend is a Python script and so tries to write a bytecode cache file. This isn't desirable in any case, but in SELinux environments is actually forbidden and causes:

type=AVC msg=audit(1425316846.172:470): avc: denied { create } for pid=1432 comm="hpfax" name="__pycache__" scontext=system_u:system_r:cupsd_t:s0-s0:c0.c1023 tcontext=system_u:object_r:usr_t:s0 tclass=dir permissive=0

The fix is to call the Python interpreter with the -B flag.

(Incidentally, is there a reason this script is not run in Python3 like everything else?)