use python3 instead of python

use python3 instead of python as python2 is EOL January 2020.

Signed-off-by: Scott Branden <scott.branden@broadcom.com>
Reviewed-by: Jerome Forissier <jerome@forissier.org>
Tested-by: Jerome Forissier <jerome@forissier.org> (QEMU, CFG_GCM_NIST_VECTORS=y)
diff --git a/scripts/rsp_to_gcm_test.py b/scripts/rsp_to_gcm_test.py
index 0543541..e4418be 100755
--- a/scripts/rsp_to_gcm_test.py
+++ b/scripts/rsp_to_gcm_test.py
@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 
 modes = {'encrypt': 0, 'decrypt': 1}