Update checkcolor.py hashbang to Python 3

Test: Run checkcolor.py as a repo hook
Bug: 186749906
Change-Id: Iada108e92a9c923fb4b4648549e2c536cb785117
diff --git a/checkcolor.py b/checkcolor.py
index c64495e..f1033ad 100755
--- a/checkcolor.py
+++ b/checkcolor.py
@@ -1,4 +1,4 @@
-#!/usr/bin/python
+#!/usr/bin/env python3
 
 #
 # Copyright 2017, The Android Open Source Project