[Python-3000] Nix dict.copy()

[Raymond Hettinger]

I recommend dropping the dict.copy() method from Py3.0.

* We can already write:  newd = copy.copy(d).
* We can also write:     newd = dict(d)
* Both of those approaches also work for most other containers.
* The collections.Mapping ABC does not support copy().
* copy() is not a universal feature of mapping like objects
* Lists do not have a copy() method.
* If we drop dict.copy(), I'll can also drop set.copy() which is unneeded.

Let's make the basic APIs as clean and parallel as possible.

Raymond