Avoid recommending Self usages in metaclasses (#8639)
PEP 673 forbids the use of `typing(_extensions).Self` in metaclasses, so
we want to avoid flagging `PYI034` on metaclasses. This is based on an
analogous change in `flake8-pyi`:
https://github.com/PyCQA/flake8-pyi/pull/436.
Closes https://github.com/astral-sh/ruff/issues/8353.
