perf scripts python: Add Python 3 support to Core.py

Support both Python 2 and Python 3 in Core.py. This should have no
functional change.

Signed-off-by: Jeremy Cline <jeremy@jcline.org>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Herton Krzesinski <herton@redhat.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/0100016341a72ebe-e572899e-f445-4765-98f0-c314935727f9-000000@email.amazonses.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>