Add better default value for TypeVar default (PEP 696)

[cdce8p]

Ref: #77 (comment)

/CC: @Gobot1234 @JelleZijlstra
This is a solution. Let me know if you had something different in mind.

[Gobot1234]

Should the default be not be public to allow people to check if a default wasn't passed?

[cdce8p]

Should the default be not be public to allow people to check if a default wasn't passed?

That was my first idea. However, it's not actually necessary. Take a look at _DefaultMixin. We pass all inputs (expect our own default) to typing._type_check which itself calls _type_convert. If None is passed, the value stored in self.__default__ will be type(None), for everything else it's just the arg itself. So unless I'm missing something, it should already work.

https://github.com/python/cpython/blob/v3.11.0rc2/Lib/typing.py#L159-L165

[JelleZijlstra]

Might end up being 4.5.0, but we can change that later.

[srittau]

We should probably just use UNRELEASED in the future.