[issue27984] singledispatch register should typecheck its argument

[Xiang Zhang]

Xiang Zhang <angwerzx at 126.com> added the comment:

Revising the patch, register() is fixed by feature in #32227. So I will only fix dispatch() in 3.7 and 3.8, the whole patch to 3.6.

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versions: +Python 3.6

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