feat(types): add Union and Optional to typing.h

[InvincibleRMC]

Description

Add Union and Optional for better static typing on the python side.

Suggested changelog entry:

``Union`` and ``Optional`` were added to ``pybind11/typing.h``

[rwgk]

Looks useful.

I just converted this to a Draft PR.

Could you please work on the GitHub Actions failures? Then click Ready for review and maybe tag me.

[InvincibleRMC]

Will do. It seems to be falling into the default handle_type_name rather than the special case describe in typing.h but, will keep investigating.

[rwgk]

Maybe you just something like this?

https://github.com/pybind/pybind11/pull/5073/files

[InvincibleRMC]

The solution was to inherit from one of the types found in cast.h 900-1035. I wasn't super sure on the best type to inherit from but, from what I could tell type::type made some sense. If there is some better type to inherit from let me know.

[InvincibleRMC]

@rwgk the CI is now passing.

[InvincibleRMC]

With further testing on actual code this does not seem to currently work.

[InvincibleRMC]

@rwgk With some updated testing and switching to inheriting from py::object it seems to be all working.

[rwgk]

Looks good to me. I'll run this by other maintainers for a 2nd set of eyes.