ssh.py use python3

[plusls]

I found that in

pwntools/pwnlib/tubes/ssh.py

Line 959 in f3b08ed

#!/usr/bin/env python

pwntools use python in remote host, but now python2 is deprecated, I think pwntools should check ssh remote host does the remote machine has python3, if it has python3, the script should use python3 first.