Update path for the python binary

Changes the python binary to /usr/bin/python3 instead of the environment
path.
diff --git a/thalerj/openbmctool.py b/thalerj/openbmctool.py
index 5d220b2..6446d34 100644
--- a/thalerj/openbmctool.py
+++ b/thalerj/openbmctool.py
@@ -1,4 +1,4 @@
-#!/usr/bin/env python3
+#!/usr/bin/python3
 """
  Copyright 2017 IBM Corporation