Prefer Python3 by default.

run

 #!/bin/sh
-if python2 -V 2>/dev/null; then
-	exec python2 -m "sshuttle" "$@"
+if python3 -V 2>/dev/null; then
+	exec python3 -m "sshuttle" "$@"
 else
 	exec python -m "sshuttle" "$@"
 fi