Move to python3.

I tried it during Interop Tokyo ShowNet 2018 on the python3-tmp branch, but it was not a suitable way. It may need cleanup and refactoring.

When used keyboard interruption with Ctrl-c, called traceback by python like following error.
I think daedoman needs exception handler.

upa-san
how do you think about this?
Is there some suggestion?

~/C/deadman ❯❯❯ ./deadman deadman.conf
Traceback (most recent call last):
File "./deadman", line 593, in
curses.wrapper (main)
File "/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/curses/wrapper.py", line 43, in wrapper
return func(stdscr, _args, *_kwds)
File "./deadman", line 558, in main
deadman.main ()
File "./deadman", line 478, in main
time.sleep (PING_ALLTARGET_INTERVAL)
KeyboardInterrupt

~/C/deadman ❯❯❯ ./deadman deadman.conf
Traceback (most recent call last):
File "./deadman", line 593, in
curses.wrapper (main)
File "/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/curses/wrapper.py", line 43, in wrapper
return func(stdscr, _args, *_kwds)
File "./deadman", line 558, in main
deadman.main ()
File "./deadman", line 471, in main
target.send ()
File "./deadman", line 85, in send
res = self.ping.send ()
File "./deadman", line 186, in send
result = commands.getoutput (pingcmd)
File "/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/commands.py", line 50, in getoutput
return getstatusoutput(cmd)[1]
File "/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/commands.py", line 60, in getstatusoutput
text = pipe.read()
KeyboardInterrupt

The current implementation uses ping commands on OSes. However, their options are different and sometimes changed (e.g., on ubuntu 18.04). So, we need to use OS-independent ping.

Which is a better way for achieving it?

when macOS adopts python3, I will start refactor.