Backport type annotation syntax support for older python versions

[jcrist]

          Pydantic handles this via https://github.com/alexmojaki/eval_type_backport -- is there any appetite for doing the same in msgspec? Happy to PR it if that it'd be supported.

Originally posted by @ashb in #737 (comment)

[jcrist]

A few options I see here:

• Do as pydantic does (use eval_type_backport if installed, error otherwise and suggest the user install it). This keeps it as an optional dependency.
• Add it as a mandatory dependency for older python versions. This would be our first mandatory dependency though, so I'm -0.5 on that.
• Write our own implementation (looking at their code, I think there's a simpler way we could add support here). Downside here is this is all hacks and it'd be hacks we'd maintain rather than someone else.

I'd be open to either option 1 or 3. Thoughts @ashb? Is this something you'd be interested in contributing?

[ashb]

In terms of effort I can spare l make time to do no.1, not sure I've got the spare cycles to do a full implementation.

Happy to pr approach 1

[jcrist]

Sounds good to me, thanks!

[ashb]

@jcrist Re testing: am I right in thinking that the cibuildwheel will test against multiple versions of python, so do I need to convice the GHA pipeline to run 3.9 with and without the dep installed?

[jcrist]

There are ways to hack in testing without it I installed that wouldn't require another ci build - if you could just write tests for behavior with it installed I can add/fixup those afterward.