dict_update(): Generalize this method so {}.update() accepts any

dict_update(): Generalize this method so {}.update() accepts any
"mapping" object, specifically one that supports PyMapping_Keys() and
PyObject_GetItem().  This allows you to say e.g. {}.update(UserDict())

We keep the special case for concrete dict objects, although that
seems moderately questionable.  OTOH, the code exists and works, so
why change that?

.update()'s docstring already claims that D.update(E) implies calling
E.keys() so it's appropriate not to transform AttributeErrors in
PyMapping_Keys() to TypeErrors.

Patch eyeballed by Tim.