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Michael Chermside had a good point -- the hook should be
called __missing__ (he suggested __on_missing__ but I think
__missing__ is fine).

Here's a new patch, which also implements and documents and
tests the change that I earlier agreed to on python-dev:
dict doesn't define on_missing (__missing__) but just checks
for it -- if a subclass doesn't define it, the default
behavior applies.

__missing__ must be implemented as a method -- it can't be
an instance variable.