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 <john.garry@huawei.com>
Signed-off-by: Ian Rogers <irogers@google.com>
Tested-by: John Garry <john.garry@huawei.com>
Tested-by: Thomas Richter <tmricht@linux.ibm.com>
Tested-by: Xing Zhengjun <zhengjun.xing@linux.intel.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Ananth Narayan <ananth.narayan@amd.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Andrew Kilroy <andrew.kilroy@arm.com>
Cc: Caleb Biggers <caleb.biggers@intel.com>
Cc: Felix Fietkau <nbd@nbd.name>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: James Clark <james.clark@arm.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Kajol Jain <kjain@linux.ibm.com>
Cc: Kan Liang <kan.liang@linux.intel.com>
Cc: Kshipra Bopardikar <kshipra.bopardikar@intel.com>
Cc: Like Xu <likexu@tencent.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Mathieu Poirier <mathieu.poirier@linaro.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Nick Forrington <nick.forrington@arm.com>
Cc: Paul Clarke <pc@us.ibm.com>
Cc: Perry Taylor <perry.taylor@intel.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Qi Liu <liuqi115@huawei.com>
Cc: Ravi Bangoria <ravi.bangoria@amd.com>
Cc: Sandipan Das <sandipan.das@amd.com>
Cc: Santosh Shukla <santosh.shukla@amd.com>
Cc: Stephane Eranian <eranian@google.com>
Cc: Will Deacon <will@kernel.org>
Link: https://lore.kernel.org/r/20220629182505.406269-2-irogers@google.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>