Choose an appropriate python version for x.py at runtime

- Default to python3 -> python -> python2
- Start as shell script and re-execute the program with the right python
  interpreter

Author: jyn514

x.py

@@ -1,4 +1,18 @@
-#!/usr/bin/env python
+# The beginning of this script is both valid shell and valid python, such that
+# the script starts with the shell and is reexecuted with the right python.
+# This works because shells only execute a line at a time.
+# Thanks to `./mach` from servo for the idea!
+''':' && {
+exists() { command -v "$1" >/dev/null 2>&1; }
+if exists python3; then
+    exec python3 "$0" "$@"
+elif exists python; then
+    exec python "$0" "$@"
+else
+    exec python2 "$0" "$@"
+fi
+}
+'''
 
 # This file is only a "symlink" to bootstrap.py, all logic should go there.