Enforce Python 3 for unittests

This commit updates all tests to enforce the Python3 executable. This is
necessary because the `assertRegex` function we use was renamed to this
name only in Python 3.2 [1]

For reference:
s;/usr/bin/env python;/usr/bin/env python3;g

[1]: https://docs.python.org/3/library/unittest.html#unittest.TestCase.assertRegex

test/sorting.t

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 # -*- coding: utf-8 -*-
 ###############################################################################
 #