perf python: Prefer python3

perf python: Prefer python3

The PYTHON_AUTO code orders the preference for the PYTHON command to be
python3, python and then python2. python3 makes a more logical
preference as python2 is no longer supported:

  https://www.python.org/doc/sunset-python-2/

Reorder the priority of the PYTHON command to be python2, python and
then python3.

Reported-by: John Garry <[email protected]>
Signed-off-by: Ian Rogers <[email protected]>
Tested-by: John Garry <[email protected]>
Tested-by: Thomas Richter <[email protected]>
Tested-by: Xing Zhengjun <[email protected]>
Cc: Alexander Shishkin <[email protected]>
Cc: Ananth Narayan <[email protected]>
Cc: Andi Kleen <[email protected]>
Cc: Andrew Kilroy <[email protected]>
Cc: Caleb Biggers <[email protected]>
Cc: Felix Fietkau <[email protected]>
Cc: Ingo Molnar <[email protected]>
Cc: James Clark <[email protected]>
Cc: Jiri Olsa <[email protected]>
Cc: Kajol Jain <[email protected]>
Cc: Kan Liang <[email protected]>
Cc: Kshipra Bopardikar <[email protected]>
Cc: Like Xu <[email protected]>
Cc: Mark Rutland <[email protected]>
Cc: Mathieu Poirier <[email protected]>
Cc: Namhyung Kim <[email protected]>
Cc: Nick Forrington <[email protected]>
Cc: Paul Clarke <[email protected]>
Cc: Perry Taylor <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Qi Liu <[email protected]>
Cc: Ravi Bangoria <[email protected]>
Cc: Sandipan Das <[email protected]>
Cc: Santosh Shukla <[email protected]>
Cc: Stephane Eranian <[email protected]>
Cc: Will Deacon <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Arnaldo Carvalho de Melo <[email protected]>