Change the shebang used for python

The shebang `/usr/bin/python3` assumes python3 is installed in /usr/bin,
which isn't always true (e.g., in the manylinux docker image).  Using
`/usr/bin/env python3` uses the python3 that is in the path, which is
more flexible.

src/relative_to.py

@@ -1,4 +1,4 @@
-#!/usr/bin/python3
+#!/usr/bin/env python3
 
 import sys
 from os import path