Added PyDict_CheckExact() in revisions 53655 and 53656.  A side-effect of this change is to slow-down updates with dict subclasses that are not overriding keys() and __getitem__().  This is especially unfortunate given good existing alternatives and given a lack of real use cases (dict subclasses that aspire to hand-off updates but not use their actual keys and mapped values).

Left out the gratuitous expansion of the API which exposed too much of the internal implementation and sought to introduce more implicit behaviors that would better be handled by explictly passing in an iterable of items to d.update().  For example.  d.update((k(x), g(x)) for x in myweirdmapping).