DEP: Re-enable PyPy wheel builds

[djhoese]

PyPy wheel builds were turned off in cibuildwheel 3+ by default. Most of them were already being skipped with:

CIBW_SKIP: "pp*-win* pp31*"

These skips were last modified from what I can tell in 2023. If someone needs PyPy builds and has the time then the following should be added/modified in the CI config (release.yaml):

CIBW_ENABLE: cpython-freethreading pypy

That is, add pypy to the optional features of cibuildwheel. Then builds can be tested and try to resolve any issues that arise.