Use python3 in shebangs.

Should fix #1789.

amalgamate.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 """A package-based, source code amalgamater."""
 import os
 import sys

scripts/xonsh

@@ -1,3 +1,3 @@
-#!/usr/bin/env python -u
+#!/usr/bin/env python3 -u
 from xonsh.main import main
 main()

setup.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 # -*- coding: ascii -*-
 """The xonsh installer."""
 # Note: Do not embed any non-ASCII characters in this file until pip has been

xonsh-in-docker.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 import subprocess
 import os
 import argparse