[issue18986] Add a case-insensitive case-preserving dict

[Antoine Pitrou]

Antoine Pitrou added the comment:

> I also note that defaultdict is implemented via a special method on
> dict itself (__missing__), and if this one was implemented the same
> way it would be easy to combine the features.

It's not that simple: to remember the original casing you need either a
second container, or to use (original_key, value) tuples as values. Both
approaches have non-trivial repercussions on the implementation of many
methods.

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