I think we should also be testing things with some different sets of attributes specified
 - e.g. ```PartialDateTime(month=4) != PartialDateTime(month=4, day=7)```  (was previously 'True')
 - and the other way around (left-right reverse), (was previously 'False' -- i.e. not the same)

These are the actual cases that previously did unexpected things
 -- in particular, PartialDateTime() would == any instance at all (but not the reverse).