Promote pythoncapi-compat project

[gvanrossum]

@vstinner maintains a repository (https://github.com/python/pythoncapi-compat) with backwards compatibility shims that allow extensions to use certain new C APIs while running on older Python versions.

We could officially promote this approach as a way for projects to be able to move forward while still supporting older Python versions.

I currently don't know what the limitations are -- possibly this doesn't work with the Stable ABI?

Credit-To: @ericsnowcurrently who mentioned this in our team meeting today.

[encukou]

The other limitation is that it only works for C. We might want to build an actual linkable library.

[encukou]

An idea @colesbury floated at the sprint (from one of the HPy devs? I didn't catch which one):
Make this a pip-installable library, with the functions it provides exported in a capsule, plus a header file with shims that import this capsule and call the functions in it.

[vstinner]

The other limitation is that it only works for C. We might want to build an actual linkable library.

I wrote the deadparrot project which is implemented as a C shared library.

I'm not sure if it can be used by other programming languages since it renames PyXXX functions to DeadPyXXX. Well, the project is still experimental :-)