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/serialposix.py b/serial/serialposix.py
index 3ac6283..82fe59a 100644
--- a/serial/serialposix.py
+++ b/serial/serialposix.py
@@ -26,6 +26,8 @@
 # - aix (AIX)               /dev/tty%d
 
 
+from __future__ import absolute_import
+
 # pylint: disable=abstract-method
 import errno
 import fcntl