Support Python 3

[juergenhoetzel]

dict.iteritems() was removed in Python 3.0.

Fixes:

 → ./psig  -p $(pidof emacs)
Traceback (most recent call last):
  File "/home/juergen/ghq/github.com/erikdw/psig/./psig", line 229, in <module>
    main(args)
  File "/home/juergen/ghq/github.com/erikdw/psig/./psig", line 177, in main
    print_signal_info(args, args.pid, max_pid_length)
  File "/home/juergen/ghq/github.com/erikdw/psig/./psig", line 157, in print_signal_info
    for k,v in obtain_signal_info(pid).iteritems():
AttributeError: 'collections.OrderedDict' object has no attribute 'iteritems'