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 <tonyj@suse.de>
Cc: Paolo Bonzini <pbonzini@redhat.com> <pbonzini@redhat.com>
Link: http://lkml.kernel.org/r/20190222230619.17887-12-tonyj@suse.deSigned-off-by: Seeteena Thoufeek <s1seetee@linux.vnet.ibm.com>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>