instead of every integer from 0 to 400. Exhaustive testing wastes time without
providing enough more assurance that the code is correct.

it was released, but that reversion never made it to the Py2.6 head.

With some changes of my own thrown in (e.g. backport of r58107).

the tuple isn't used as the "exception arguments tuple".

  - The copy module now "copies" function objects (as atomic objects).
  - dict.__getitem__ now looks for a __missing__ hook before raising
    KeyError.
  - Added a new type, defaultdict, to the collections module.
    This uses the new __missing__ hook behavior added to dict (see above).

Many of these tests are redundant, but this will ensure
that the mapping protocols all stay in sync.
Also, added a test for dictionary subclasses.