Add Void type

[ocket8888]

Add Void Type

bpo-43242: Adds a "Void" type to the typing library, which indicates (to type checkers) a return type that cannot be used in a valid assignment.

[the-knights-who-say-ni]

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[gvanrossum]

This should be discussed on typing-sig first.

[ocket8888]

What's "typing-sig"?

[merwok]

The mailing list for people interested in typing (SIG is special interest group, meaning a subset of the big python community): https://mail.python.org/archives/list/typing-sig@python.org/