Here's a work-in-progress patch that uses OrderedDict as the default class definition namespace (instead of dict).  While the actual class namespace is a dict, the definition order is preserved on a new attribute on class objects: __definition_order__.  This is useful for cases such as class decorators that would make use of the definition order to post-process the class.  This is something that Guido okay'ed a while back.

It may be a stretch to get this into 3.5, but the final change should be pretty small.  It also depends on the C implementation of OrderedDict (see issue #16991) which has not landed and may not make it in time.

Note that the patch is still a bit buggy, partly due to bugs in the C OrderedDict patch.  Also, the attached patch is based off the C OrderedDict patch, so I expect reitveld won't be able to handle it.  The code on which the patch is based may be found in the "class-namespace" branch of https://pypi.python.org/pypi/cyordereddict.