Add missing sections to blurbs

[hugovk]

Fixes python/core-workflow#525.

After adding more validation to blurb in python/core-workflow#507 (unreleased), blurb merge fails for these three files because they're missing a section entry.

Other "no new changes" blurbs do contain a section: Library entry:

• https://github.com/python/cpython/blob/main/Misc/NEWS.d/3.5.4.rst?plain=1
• https://github.com/python/cpython/blob/main/Misc/NEWS.d/3.5.5.rst?plain=1
• https://github.com/python/cpython/blob/main/Misc/NEWS.d/3.6.4.rst?plain=1
• https://github.com/python/cpython/blob/main/Misc/NEWS.d/3.6.6.rst?plain=1

We could modify blurb to allow files in Misc/NEWS.d/next to skip this validation, but it's easier to add the missing sections.

[hugovk]

We should also backport this back to 3.8, so blurb merge works for security releases too.

RMs @pablogsal @ambv, is that okay?

(This would only be a problem when the next version of blurb is released, and it's not imminently planned.)

[serhiy-storchaka]

Let just create backport and left the on @pablogsal and @ambv. I think they missed your question, but it will be more difficult to miss PRs.

[miss-islington-app]

Thanks @hugovk for the PR 🌮🎉.. I'm working now to backport this PR to: 3.8, 3.9, 3.10, 3.11, 3.12.
🐍🍒⛏🤖

[bedevere-app]

GH-115336 is a backport of this pull request to the 3.12 branch.

[bedevere-app]

GH-115337 is a backport of this pull request to the 3.11 branch.

[bedevere-app]

GH-115338 is a backport of this pull request to the 3.10 branch.

[bedevere-app]

GH-115339 is a backport of this pull request to the 3.9 branch.

[bedevere-app]

GH-115340 is a backport of this pull request to the 3.8 branch.

[pablogsal]

We should also backport this back to 3.8, so blurb merge works for security releases too.

I am fine doing this for 3.10 and 3.11. @ambv what do you think?