Change the string representation of the bottom type from <nothing> to typing.Never #15950

Feature

Rather than showing the string <nothing>, which is unclear to mypy users, use typing.Never.

Pitch

It is not obvious to a typing user that <nothing> means the same thing as typing.Never.

I've been active in some other issues regarding a behavior which incorrectly produces <nothing>.
The originating issue which prompted my interest was a click bug (pallets/click#2558) which produced <nothing>.
I then made comments and filed issues (e.g. #15931) based on a significant misunderstanding of <nothing> (and I do apologize for burning people's time on this!).
Aligning the string from mypy with symbols in typing would have made the connection clear from the start.

Looking at how mypy renders other types from typing, I see typing.Any renders as Any, and typing.TextIO renders as typing.TextIO.
So there is precedent for either Never or typing.Never.