Add preliminary support for Python 3.13

[icemac]

Replacement for #144.

Needs:

• cffi release to be able to build persistent

[icemac]

Using uv does not help, it seems to ignore the already installed cffi when it is used in setup_requires and installs the (not compatible) released version again.

That's sad, so we have to wait for a cffi release.

[icemac]

See also python-cffi/cffi#23 (comment)

[dataflake]

Superceded by #154