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/urlhandler/protocol_socket.py b/serial/urlhandler/protocol_socket.py
index 5c77415..36cdf1f 100644
--- a/serial/urlhandler/protocol_socket.py
+++ b/serial/urlhandler/protocol_socket.py
@@ -16,6 +16,8 @@
 # options:
 # - "debug" print diagnostic messages
 
+from __future__ import absolute_import
+
 import errno
 import logging
 import select