lapack_testing.py: python2 explicit in hashbang

Use `python2` explicitly in the hashbang instead of just `python`
because Ubuntu 20.04 does not ship with a `python` executable any
longer.

Author: christoph-conrads

lapack_testing.py

@@ -1,4 +1,4 @@
-#! /usr/bin/env python
+#! /usr/bin/env python2
 # -*- coding: utf-8 -*-