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/__init__.py b/serial/__init__.py
index c24ced8..dcd7c12 100644
--- a/serial/__init__.py
+++ b/serial/__init__.py
@@ -7,6 +7,8 @@
 #
 # SPDX-License-Identifier:    BSD-3-Clause
 
+from __future__ import absolute_import
+
 import sys
 import importlib