Switch script to Python3

generate_psa_constants.py was accidentally declared with an
implicitly-Python2 shebang.
diff --git a/scripts/generate_psa_constants.py b/scripts/generate_psa_constants.py
index ab7f134..a76bf7e 100755
--- a/scripts/generate_psa_constants.py
+++ b/scripts/generate_psa_constants.py
@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 import os
 import re
 import sys