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Thanks both!  Here's a new patch:

- fixed most of Neal's nits
- fixed most of Raymond's first set of nits
- updated UserDict

Notes:

- I can't use PyObject_CallMethodObjArgs without also
  converting "on_missing" to an object.
  Right now I don't care enough to bother (especially
  since it is only called for proper subclasses).
- I can't find Py_Clear(). Raymond, what did you mean?
- It's not clear how (or even whether) to update DictMixin.
- Haven't gotten to copy() yet -- good point though!
- Still no docs.
- In addition to Raymond's explanation, it's also the case
  that NULL appears as None to the user through the magic
  of the T_OBJECT attribute machinery.