plagiarise `useful_types.SequenceNotStr`

[jorenham]

as defined in
https://github.com/hauntsaninja/useful_types/blob/9937e8b44f664426b4f1a9a8a183f20452ddc941/useful_types/__init__.py#L283-L295

and originally posted at python/typing#256 (comment)

---

Note to self: be sure to be very sneaky about it, otherwise @hauntsaninja might notice