Fix crash when running under python3.

Fix crash when running under python3.

This reverts cd6ffb5913664844290f44a7ea48533caf8c459e

Traceback (most recent call last):
  File "./iotop.py", line 12, in <module>
    main()
  File "./iotop/iotop/ui.py", line 597, in main
    main_loop()
  File "./iotop/iotop/ui.py", line 587, in <lambda>
    main_loop = lambda: run_iotop(options)
  File "./iotop/iotop/ui.py", line 485, in run_iotop
    return curses.wrapper(run_iotop_window, options)
  File "/usr/lib/python3.2/curses/wrapper.py", line 43, in wrapper
    return func(stdscr, *args, **kwds)
  File "./iotop/iotop/ui.py", line 478, in run_iotop_window
    ui.run()
  File "./iotop/iotop/ui.py", line 153, in run
    total = self.process_list.refresh_processes()
  File "./iotop/iotop/data.py", line 459, in refresh_processes
    self.processes.items() if
  File "./iotop/iotop/data.py", line 460, in <listcomp>
    process.update_stats()])
  File "./iotop/iotop/data.py", line 358, in update_stats
    for tid, thread in self.threads.items():
RuntimeError: dictionary changed size during iteration

[This is valid since 0fc4ab84c8cbba1fbe83dc71fb89100b87c54898
 added the self.threads = dict(...)]