FYI: types-typing-extensions is dead

[hauntsaninja]

As part of modular typeshed, we split off third party stubs into their own distributions (#2491). But one of these was not quite like the others: typing_extensions.

In #5233 we decided to special case and distribute typing_extensions along with the rest of the stdlib. Here's a quick summary of why:

• typing_extensions is the only thing outside of the stdlib that stdlib stubs depend on.
• Since stdlib stubs themselves aren't packaged as a distribution, there's no way to ensure that typing_extension stubs are present.
• This causes issues in cases where types-typing-extensions is not installed. These issues are extra bad, because users may not even be aware of typing_extensions and are just trying to use stdlib types.
• For a concrete example, using mypy with --python-executable pointed at a venv without types-typing-extensions would result in hard to diagnose failures
• Another example of code I've seen that would no longer check due to this issue is where people guard an typing_extensions import with if TYPE_CHECKING and thereby avoid having to install anything third party.
• typing_extensions' whole point of existence is to be special cased :-)

Relevant links:
#5229 (comment)
#5109 (comment)
#5233