Fix pickle/copy support for the missing singleton

[mattclay]

Fix pickle/copy support for the missing singleton:

• Fix pickle support for the missing singleton.
• Add unit test for pickling the missing singleton.
• Add unit test for copying the missing singleton.

Fixes #2027

[mattclay]

Returning "missing" from __reduce__ is a documented solution for singleton types:

If a string is returned, the string should be interpreted as the name of a global variable. It should be the object’s local name relative to its module; the pickle module searches the module namespace to determine the object’s module. This behaviour is typically useful for singletons.

[mattclay]

I used this form to minimize changes, but it could be replaced with an actual class definition if desired.

[davidism]

Yeah, that makes more sense. I don't see a reason a "singleton" was done this way, it's enough to call the class _MissingType to indicate it's not public.

[nitzmahone]

Always the challenge when interacting with unfamiliar projects/maintainers around finding the balance of "minimize changes" vs "if you were building this today, it would look very different" 😆