Add Python 3.14 support, experimental subinterpreter/freethreading support

[elprans]

The bulk of the changes here is a rewrite of recordobj.c to use modern
CPython API to properly isolate the module (PEP 489, PEP 573, PEP 630).
This, along with Cython flags, enables support for safely importing
asyncpg in subinterpreters. The Record freelist is now
thread-specific, so asyncpg should be thread-safe at the C level.
Support for subinterpreters and freethreading is EXPERIMENTAL.

[ddanier]

As of no pre-built package being available for asyncpg on Python 3.14 you cannot install the package on "slim" docker images, cause gcc is not installed there. This is an issue as you tend to use those "slim" variants when you care about security. Of course it is possible to get this working by either compiling this package yourself on a non-"slim" image and then use a multistage build or switching to the non-"slim" version, but that either does increase complexity or reduce security significantly.

What I want to say: I really would like this to get finished and merged soon. Thanks for all the effort! ;-)

[1st1]

Any reason for this file to be moved? No diff now, all code is like new which makes review harder

[elprans]

There were some odd Cython issues initially, but I'll see if I can move it back.

[elprans]

Done, @1st1, please take another look.

[1st1]

Can we move recordobj.c back where it was?

[elprans]

Can we move recordobj.c back where it was?

I couldn't find a way to do it. Cython does not expose a way to plug into its generated module state, so record must be a standalone module, not a concatenation. Why is this important?

[1st1]

I couldn't find a way to do it. Cython does not expose a way to plug into its generated module state, so record must be a standalone module, not a concatenation. Why is this important?

Just for me to review the thing, it's a big file. Were there any changes in it?