Stop testing Python 3.15

[icemac]

It is broken probably because we need a fix in cffi, see https://github.com/zopefoundation/Persistence/actions/runs/19610857054/job/56156444668

[dataflake]

At the root of the recurring CFFI issues is the CFFI dependency in the persistent package. I wonder why in the world that is even there.

[davisagli]

@dataflake It looks like it is used for the pure-Python implementation of the pickle cache, and was an optional dependency until zopefoundation/persistent#107. Maybe it's worth re-adding the other implementation that was removed there and making cffi optional again, since we have this recurring problem where there aren't wheels available for cffi on new Python versions as early as we'd like.

[dataflake]

@davisagli Thanks for the hint. I added a comment in zopefoundation/persistent#107 because I am just tired of the constant issues. The reasons mentioned for the change, like code simplification and test coverage in persistent, are not worth this never ending hassle.