Fix TypeError with Python 2.7 (#575)

TypeError: 'encoding' is an invalid keyword argument
for this function

setup.py

@@ -9,7 +9,7 @@ if version_tuple[3] is not None:
 else:
     version = "%d.%d.%d" % version_tuple[:3]
 
-with open('./README.rst', encoding='utf-8') as f:
+with io.open('./README.rst', encoding='utf-8') as f:
     readme = f.read()
 
 setup(