perf script python: Add Python3 support to export-to-postgresql.py

Support both Python2 and Python3 in the export-to-postgresql.py script.

The use of 'from __future__' implies the minimum supported Python2 version
is now v2.6

Signed-off-by: Tony Jones <tonyj@suse.de>
Link: http://lkml.kernel.org/r/20190309000518.2438-3-tonyj@suse.de


Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Signed-off-by: Seeteena Thoufeek <s1seetee@linux.vnet.ibm.com>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>