Remove typing._promote

[srittau]

Part of #7580

[github-actions]

Diff from mypy_primer, showing the effect of this PR on open source code:

discord.py (https://github.com/Rapptz/discord.py)
- ...typeshed_to_test/stdlib/typing.pyi:1040: note: "update" of "TypedDict" defined here
+ ...typeshed_to_test/stdlib/typing.pyi:1036: note: "update" of "TypedDict" defined here

[JelleZijlstra]

Mypy still support this (search for @_promote in its test suite), but it's undocumented and I have no idea whether anyone still uses it. (Evidently nobody in the mypy-primer corpus does, but that's not all code.) I'm fine with removing this: mypy can add it to mypy-extensions if someone still wants this feature.

[hauntsaninja]

Yes, okay to remove. There's no winning with promotion, on the one hand we have "isinstance(5, Number)" crowd and on the other we have the "(5).hex()" crowd