Add shebang to gerrit_monitoring.py script

This allows to run the script without explicitly specifying the
interpreter to use.

Change-Id: I900b2dae90a87fb6bae65c6d1549ad9d5d29cd48
diff --git a/gerrit_monitoring.py b/gerrit_monitoring.py
index 385fc87..3c78780 100644
--- a/gerrit_monitoring.py
+++ b/gerrit_monitoring.py
@@ -1,3 +1,4 @@
+#!/usr/bin/env python3
 # Copyright (C) 2020 The Android Open Source Project
 #
 # Licensed under the Apache License, Version 2.0 (the "License");