asyncpg does not install with python 3.12-dev

See [Github CI trace](https://github.com/nackjicholson/aiosql/actions/runs/4150335841/jobs/7279284210)

* **asyncpg version**: latest
* **PostgreSQL version**: latest
* **Do you use a PostgreSQL SaaS? If so, which? Can you reproduce
the issue with a local PostgreSQL install?**: No / Does not apply
* **Python version**: 3.12-dev
* **Platform**: Github CI
* **Do you use pgbouncer?**: No
* **Did you install asyncpg with pip?**: Just trying, it fails.
* **If you built asyncpg locally, which version of Cython did you use?**: 312
* **Can the issue be reproduced under both asyncio and
[uvloop](https://github.com/magicstack/uvloop)?**: I do not know.

Python 3.12, under development, seems to have changed a structure:

```
asyncpg/pgproto/pgproto.c: In function ‘__Pyx_PyInt_As_int’:
asyncpg/pgproto/pgproto.c:40800:53: error: ‘PyLongObject’ {aka ‘struct _longobject’} has no member named ‘ob_digit’
40800 | const digit* digits = ((PyLongObject*)x)->ob_digit;
| ^~
```

Also, there are two repeated warnings:

```
gcc -Wsign-compare -DNDEBUG -g -fwrapv -O3 -Wall -fPIC -I/opt/hostedtoolcache/Python/3.12.0-alpha.5/x64/include/python3.12 -c asyncpg/pgproto/pgproto.c -o build/temp.linux-x86_64-cpython-312/asyncpg/pgproto/pgproto.o -O2 -fsigned-char -Wall -Wsign-compare -Wconversion
asyncpg/pgproto/pgproto.c: In function ‘__pyx_pf_7asyncpg_7pgproto_7pgproto_11WriteBuffer_2__dealloc__’:
asyncpg/pgproto/pgproto.c:3127:5: warning: ‘ma_version_tag’ is deprecated [-Wdeprecated-declarations]
3127 | __Pyx_GetModuleGlobalName(__pyx_t_4, __pyx_n_s_BufferError); if (unlikely(!__pyx_t_4)) __PYX_ERR(1, 34, __pyx_L1_error)
| ^~~~~~~~~~~~~~~~~~~~~~~~~
In file included from /opt/hostedtoolcache/Python/3.12.0-alpha.5/x64/include/python3.12/dictobject.h:90,
from /opt/hostedtoolcache/Python/3.12.0-alpha.5/x64/include/python3.12/Python.h:61,
from asyncpg/pgproto/pgproto.c:31:
/opt/hostedtoolcache/Python/3.12.0-alpha.5/x64/include/python3.12/cpython/dictobject.h:22:34: note: declared here
22 | Py_DEPRECATED(3.12) uint64_t ma_version_tag;
| ^~~~~~~~~~~~~~

/opt/hostedtoolcache/Python/3.12.0-alpha.5/x64/include/python3.12/internal/pycore_code.h:434:41: warning: conversion to ‘int’ from ‘unsigned int’ may change the sign of the result [-Wsign-conversion]
434 | return adaptive_counter_bits(value, backoff);
| ^~~~~~~
```

Start with the linked GitHub CI trace and the failing generated file asyncpg/pgproto/pgproto.c, reproducing installation with Python 3.12-dev. Investigate the reported PyLongObject ob_digit compilation error and verify that asyncpg installs successfully on that Python version without the reported build failure.

Written by the indexing model from the issue text.