CI: Ensure PRs are correctly mentioned in the release notes

[leofang]

We want to automate this so that we don't need to revisit all past PRs by release time.

During a recent offline discussion, https://sphinx-github-changelog.readthedocs.io/en/latest/ was suggested, but upon a closer inspection it does not fit with our release model.

If we cannot automate this, at least we should have a way to ensure PR authors to update the release note entry.

[rwgk]

pybind11 has a PR template with a "Suggested changelog entry" section near the bottom. Henry wrote a Python script that harvests those. This simple approach works pretty well in practice.