issue7989a.diff is a partial success implementing Nick Coghlan's testing idea.  Unfortunately, datetime implementation with it's circular dependency on _strftime is not very robust with respect to import trickery.

I am calling this a partial success because although running Lib/test/test_datetime.py does not report any errors, it only works with pure python version of pickle. (I had to add sys.modules['_pickle'] = None at the top of the module to make it work.)

Also, the resulting test_datetime is quite an abomination!