Explicitly request Python 3

It varies between systems if /usr/bin/python is Python 2.x or Python
3.x. Since we now only work with Python 3.x we should be more explicit
in our scripts and documentation.

run

@@ -1,4 +1,4 @@
 #!/usr/bin/env sh
 set -e
 cd "$(dirname "$0")"
-exec python -m websockify "$@"
+exec python3 -m websockify "$@"