Replaced deprecated Py_FileSystemDefaultEncoding for Python >= 3.12

[radarhere]

For #6941.

Python 3.12 reports a deprecation - https://github.com/python-pillow/Pillow/actions/runs/5105776867/jobs/9177570491#step:7:234

warning: 'Py_FileSystemDefaultEncoding' is deprecated

https://docs.python.org/3.12/whatsnew/3.12.html#id4 instructs us to replace it with PyConfig.filesystem_encoding.

Using https://docs.python.org/3.12/c-api/init_config.html#example as a basis, I made that change.

However, I found this failed on PyPy, with

error: use of undeclared identifier 'PyConfig'

I asked in https://foss.heptapod.net/pypy/pypy/-/issues/3946, and PyPy has chosen not to implement PyConfig yet. So I've switched to the new code only for Python 3.12 and later, and we can see what happens once PyPy does support Python 3.12.