tools/power/x86/intel_pstate_tracer: python minimum version

Change the minimum python version from 2.7 to 3.6.
Remove a 2.X backwards compatibility line.

Signed-off-by: Doug Smythies <dsmythies@telus.net>
Acked-by: Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>
Reviewed-by: Mario Limonciello <mario.limonciello@amd.com>
Tested-by: Swapnil Sapkal <Swapnil.Sapkal@amd.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>