Clarification on pypy support

[black-sliver]

The classifiers on pypi and in setup.py list pypy, however that support seems to not have been updated in a while.

https://github.com/black-sliver/pony/actions/runs/10568764714 shows that pypy2.7 still works, but pypy3.8-3.10 seems to be broken. I have not tested any 3.x before 3.8.
Officially supported by pypy seems to be 2.7, 3.9 and 3.10.

Should it be documented that only pypy2.7 is supported and pypy3.x is not or should it be dropped altogether?

(The project I am working on may or may not use pypy3.x at some point, but I think the code paths that use pony will probably never switch away from cpython, so I don't plan on fixing pypy3.x myself.)