`towncrier check` should not fail on revert commits that *remove* a news fragment

[Glyph]

If you're reverting a PR to un-do a change that removes the newsfragment before a release so it isn't in the changelog, the PR should succeed its towncrier newsfragment check

[Adi Roiban]

Thanks for the report

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Just some extra info

There is also the release PR that removes the news fragments.

For the release PR the check is skipped based on the fact that the commit changes the NEWS.rst file.

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In theory, we can merge the skip check for both release and revert PRs.

I would prefer to have separate checks.

The idea is that for a release PR you want to make sure there are no release fragment files left on the branch.