The layout for python integers changed in python 3.12.
We add a module `sage.cpython.pycore_long` which copies the new
(internal) api of PyLong from python 3.12. We also implement fallback
version of these functions suitable for python pre-3.12.

Note the files implementing the `pycore_long` module (`pycore_long.pxd`
and `pycore_long.h`) are shared with fpylll and cypari2.

In python 3.12, the `struct atexit_callback` was renamed to
`struct atexit_py_callback`. The easiest workaround is to add
`#define atexit_callback atexit_py_callback` in the right place when
building using python 3.12 or newer.

Tracing has changed in python 3.12 in such a way that cython doesn't
support it properly anymore. This one file sets `profile=true` for
cython which won't work anymore (and it fails to build, at least with
cython 0.29). We just disable that line.

See also: cython/cython#5450

To use some (internal) declarations related to dict type, we have to
include `<internal/pycore_dict.h>` which needs `#define Py_BUILD_CORE`
to be loaded. This causes trouble when `Python.h` was included before
defining `Py_BUILD_CORE`, due to a macro `_PyGC_FINALIZED`. We fix it
by undefining said macro.

Some changes in ast, the old `node.n` and `node.s` are deprecated in
favour of a common `node.value`. Making this change seems better than
just ignoring the deprecation warning.

This adds some filterwarnings that trigger with python 3.12.

 - deprecation of `datetime.datetime.utcfromtimestamp()` this is
   triggered by python modules `dateutil` and `sphinx`.
 - `os.fork()` and `os.ptyfork()` are deprecated when running
   multi-threaded; I don't see an easy way out of this, so ignore it.
 - itertools won't support pickling in python 3.14; let's ignore this
   for now with the hope there's an alternative before 3.14.

Is deprecated, and it can be replaced just fine with
`importlib.util.find_spec()`.

Since `importer.find_module(...)` was removed in 3.12.

We just follow the suggestion from

https://docs.python.org/3/library/importlib.html#importing-a-source-file-directly

Note that the last three added lines here could be replaced instead by

    spec.loader.load_module(module_name)

which works; however this is deprecated so it's better to use the
recommended way using `importlib.util.module_from_spec(...)` and
`spec.loader.execute_module(...)`.

In python < 3.12 we have

    sage: a = delta_qexp(1000)
    sage: sum(a[n]/float(n)^14 for n in range(1,1000))
    0.9985830631627459

This changed in python 3.12 to

    sage: sum(a[n]/float(n)^14 for n in range(1,1000))
    0.9985830631627461

The latter is the correct one as can be seen using rationals:

    sage: float(sum(a[n]/n^14 for n in range(1,1000)))
    0.9985830631627461

As a workaround, we do the sum in reverse (from small to large terms),
which gives the correct result in any case:

    sage: sum(a[n]/float(n)^14 for n in reversed(range(1,1000)))
    0.9985830631627461

In python 3.12 the printing of OrderedDict has been changed.

As of Python 3.7, regular dicts are guaranteed to be ordered, so it's
safe to replace OrderedDict by dict.

Maybe convenient to replace other uses of OrderedDict, although this is
out of scope of python 3.12 support.

Running

    sage: g = Polyhedron().face_generator()
    sage: g.__next__()
    A -1-dimensional face of a Polyhedron in ZZ^0
    sage: g.__next__()

is supposed to raise `StopIteration`. However in python 3.12 the second
call to `__next__()` leads to a crash.

This is caused by a `return -1` in `next_face_loop()` which is supposed
to mean `raise StopIteration`. Using raise explicitly fixes the crash.