Add guidance suggesting to focus on 3.14t support

[ngoldbaum]

CFFI won't support 3.13t so that will be a blocker for a lot of packages. There are also a lot of issues in 3.13t that won't be fixed as well as a few behavior differences that make it easier to write thread-safe extensions using 3.14t.

Our team agrees that it makes sense to focus on 3.14t support in packages that don't yet have support.

If so, we should add guidance suggesting all that.

Ping @colesbury, does that approach make sense to you?

[colesbury]

Yes, I think that makes sense.

CFFI won't support 3.13t so that will be a blocker for a lot of packages

If this is a major blocker to enabling packages, we can revisit the CFFI 3.13t support. But I think we want to get packages supporting 3.14t anyways.