Use absolute import everywhere

This prevents Python 2.7 from attempting to import system modules
like sys and ctypes from the 'serial' directory.
diff --git a/serial/serialutil.py b/serial/serialutil.py
index 7d51752..e847a6a 100644
--- a/serial/serialutil.py
+++ b/serial/serialutil.py
@@ -7,6 +7,8 @@
 #
 # SPDX-License-Identifier:    BSD-3-Clause
 
+from __future__ import absolute_import
+
 import io
 import time