Update art_apex_test.py to use python3.

Note that the interpreter specification in that script is
irrelevant. The `python_binary_host` definition in the
Android.bp defines what interpreter is actually used.

Test: m art-check-release-apex-gen
Change-Id: I9abd12a1956fabe415ce3e06422022a3975f2b34
diff --git a/build/apex/art_apex_test.py b/build/apex/art_apex_test.py
index 87be4b3..3eeb7fc 100755
--- a/build/apex/art_apex_test.py
+++ b/build/apex/art_apex_test.py
@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 # -*- coding: utf-8 -*-
 
 # Copyright (C) 2019 The Android Open Source Project