gh-98680: Add PyBUF_* constants to the Limited API listing (GH-100018)

``PyBUF_*`` constants are marked as part of Limited API of Python 3.11+.
These were available in 3.11.0 with `Py_LIMITED_API` defined for 3.11,
and are necessary to use the buffer API. Omitting them in `stable_abi.toml`
was a mistake.