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/rs485.py b/serial/rs485.py
index 2939350..d7aff6f 100644
--- a/serial/rs485.py
+++ b/serial/rs485.py
@@ -13,6 +13,8 @@
 NOTE: Some implementations may only support a subset of the settings.
 """
 
+from __future__ import absolute_import
+
 import time
 import serial