perf script python: Add Python3 support to net_dropmonitor.py

perf script python: Add Python3 support to net_dropmonitor.py

Support both Python2 and Python3 in the net_dropmonitor.py script

There may be differences in the ordering of output lines due to
differences in dictionary ordering etc.  However the format within lines
should be unchanged.

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

Signed-off-by: Tony Jones <tonyj@suse.de>
Acked-by: Neil Horman <nhorman@tuxdriver.com>
Link: http://lkml.kernel.org/r/20190222230619.17887-9-tonyj@suse.de
Signed-off-by: Seeteena Thoufeek <s1seetee@linux.vnet.ibm.com>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>