Add #! line to stm32f429i test runner.

Change-Id: Ida76b2683b6a24e9c60f2fa943672b9c5ed2f23d

targets/stm32f429i-disc1/py/stm32f429i_disc1_utils/unit_test_runner.py

diff --git a/targets/stm32f429i-disc1/py/stm32f429i_disc1_utils/unit_test_runner.py b/targets/stm32f429i-disc1/py/stm32f429i_disc1_utils/unit_test_runner.py
index a3698f6..5f125ff 100755
--- a/targets/stm32f429i-disc1/py/stm32f429i_disc1_utils/unit_test_runner.py
+++ b/targets/stm32f429i-disc1/py/stm32f429i_disc1_utils/unit_test_runner.py
@@ -1,3 +1,4 @@
+#!/usr/bin/env python3
 # Copyright 2019 The Pigweed Authors
 #
 # Licensed under the Apache License, Version 2.0 (the "License"); you may not