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_hwgrep.py b/serial/urlhandler/protocol_hwgrep.py
index 49bbebe..1a288c9 100644
--- a/serial/urlhandler/protocol_hwgrep.py
+++ b/serial/urlhandler/protocol_hwgrep.py
@@ -20,6 +20,8 @@
 # n=<N>     pick the N'th entry instead of the first one (numbering starts at 1)
 # skip_busy tries to open port to check if it is busy, fails on posix as ports are not locked!
 
+from __future__ import absolute_import
+
 import serial
 import serial.tools.list_ports