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/tools/hexlify_codec.py b/serial/tools/hexlify_codec.py
index 1371da2..bd8f6b0 100644
--- a/serial/tools/hexlify_codec.py
+++ b/serial/tools/hexlify_codec.py
@@ -18,6 +18,8 @@
 
 """
 
+from __future__ import absolute_import
+
 import codecs
 import serial