perf script python: Add Python3 support to stackcollapse.py

Support both Python2 and Python3 in the stackcollapse.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 <[e-mail]>
Cc: Paolo Bonzini <[e-mail]> <[e-mail]>
Link: http://lkml.kernel.org/r/[e-mail]
Signed-off-by: Seeteena Thoufeek <[e-mail]>
Signed-off-by: Arnaldo Carvalho de Melo <[e-mail]>

tools/perf/scripts/python/stackcollapse.py

···
 # Written by Paolo Bonzini <[e-mail]>
 # Based on Brendan Gregg's stackcollapse-perf.pl script.
 
+from __future__ import print_function
+
 import os
 import sys
 from collections import defaultdict
···
     lines[stack_string] = lines[stack_string] + 1
 
 def trace_end():
-    list = lines.keys()
-    list.sort()
+    list = sorted(lines)
     for stack in list:
-        print "%s %d" % (stack, lines[stack])
+        print("%s %d" % (stack, lines[stack]))