Clarify newsfragment requirement

I've opened several PRs recently that have no user-facing changes so I don't think there is anything to add to the changelog, but they don't seem to fit any of the excluded categories. https://github.com/PyO3/pyo3/blob/6aadaf24e4f9f4109531a048082aca89b999912b/noxfile.py#L1078-L1082 My PRs have been mostly WIP changes (prep for `no_std`) and a bit of refactoring with no user-visible changes.