use the recommended yaml.safe_load function

reason: with pyyaml >= 5, the load function without Loader is deprecated.
Even if we do not use version 5 for now, we should use good practices.
https://pyyaml.org/wiki/PyYAMLDocumentation#loading-yaml
yaml/pyyaml#257

integration_tests/suite/test_password_reset.py

@@ -36,7 +36,7 @@ def test_password_reset(self, bar, foo):
 
     def _update_password_from_email(self, raw_email, password):
         headers, body = raw_email.split('\n\n', 1)
-        email_fields = yaml.load(body)
+        email_fields = yaml.safe_load(body)
 
         token = email_fields['token']
         user_uuid = email_fields['user_uuid']