Tweak how python/python3 is used.
The kokoro Mac configs seem to be pretty old without python3, tweak how things
are invoked to try and better deal with those configs.
diff --git a/objectivec/DevTools/pddm_tests.py b/objectivec/DevTools/pddm_tests.py
index a6571f6..d5b88c9 100755
--- a/objectivec/DevTools/pddm_tests.py
+++ b/objectivec/DevTools/pddm_tests.py
@@ -1,4 +1,4 @@
-#! /usr/bin/python3
+#! /usr/bin/env python3
#
# Protocol Buffers - Google's data interchange format
# Copyright 2015 Google Inc. All rights reserved.
