Update python code for python 3.8 or higher compatibility

Changes:
     - Update shebang directive to use python3 explicitly.
     - remove preexec_fn feature which is deprecated in higher
       python version 3.8 and above.
     - bufsize fix, from documentation bufsize 1 means line buffered
      (only usable if universal_newlines=True i.e., in a text mode)
      refer: https://docs.python.org/3/library/subprocess.html
     - Fix SyntaxWarning: "is" with "=="

Change-Id: Ib96c1ba8c2327d6774d0d731b48ca7e61a4a297a
Signed-off-by: George Keishing <gkeishin@in.ibm.com>
diff --git a/lib/var_stack.py b/lib/var_stack.py
index 0b73b91..3ea3813 100644
--- a/lib/var_stack.py
+++ b/lib/var_stack.py
@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 
 r"""
 Define the var_stack class.