Minor code cleanups in arc_python.py

Remove unused library re and associated variable kstat_pobj. Add note
to documentation at start of program about required support for old
versions of Python. Change variable "format" (which is a built-in
function) to "fmt".

Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Scot W. Stevenson <scot.stevenson@gmail.com>
Closes #6869