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

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.

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