Test that generate_test_code.py is still compatible with Python 2

When Mbed TLS 2.16 was released, the requirement was Python 2, not
Python 3. Since then, upstream Python 2 support has stopped, but it is
still maintained in some long-term-support distributions. For the sake
of users who build the unit tests in such environments, test that
generate_test_code.py remains compatible with Python 2.

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
diff --git a/tests/scripts/generate_test_code.py b/tests/scripts/generate_test_code.py
index f452b37..b74ed55 100755
--- a/tests/scripts/generate_test_code.py
+++ b/tests/scripts/generate_test_code.py
@@ -1,4 +1,7 @@
 #!/usr/bin/env python3
+
+# This script should still be compatible with Python 2 in Mbed TLS 2.16.x.
+
 # Test suites code generator.
 #
 # Copyright The Mbed TLS Contributors