Refer explicitly to python3 in shebang line

On Ubuntu 20.04, there is no more python or python2. Currently I get
this: /usr/bin/env: ‘python’: No such file or directory

Change-Id: Ib310b8aa7c1bd62973ef3cc8bcaf571831ad4435

rocm_agent_enumerator

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 
 import os
 import re