tools/power/x86/intel_pstate_tracer: changes for python 3 compatibility

Some syntax needs to be more rigorous for python 3.
Backwards compatibility tested with python 2.7

Signed-off-by: Doug Smythies <dsmythies@telus.net>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>