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.