tool-recommendations: update Trusted Publisher providers #1668
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The docs currently say that only GitHub Actions is supported, but this is no longer true.
I've updated the section to include the currently supported providers, as well as include a rough date so that future readers can infer how old (and therefore stale) the list may be.
(I also added a
.. warningcallout to a statement that looked like it needed one. But I can undo that if desired!)CC @webknjaz
📚 Documentation preview 📚: https://python-packaging-user-guide--1668.org.readthedocs.build/en/1668/