Specifically request Python 3

If Sphinx is Python 3-only, Sphinx's tooling should be Python 3-only
too.

Signed-off-by: Stephen Finucane <stephen@that.guru>

tests/roots/test-apidoc-toc/mypackage/main.py

@@ -1,4 +1,5 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
+
 import os
 
 import mod_resource