docs: Drop sphinxcontrib-openapi fork, switch back to upstream

[qmonnet]

Once upon a time, Cilium docs used the openapi Sphinx add-on to generate its API reference based on the code. And things were good.

One day, Dependabot raised a security alert, stating that Mistune v2.0.2 was vulnerable to catastrophic backtracking - this is a regex parsing thing. Mistune was a dependency to m2r, an add-on to parse Markdown in Sphinx, which in turn was a dependency to openapi.

The easy path would have been to update m2r to use the latest, fixed Mistune version; but m2r was incompatible with Mistune >= 2.0.0, and also it was no longer in development.

There was a fork, m2r2, which had little activity, and would avoid the security issue by very simply pinning the Mistune version to 0.8.4 (which would either fail to build Cilium's reference correctly, or bring some incompatibilities with other dependencies, at this point the narrator does not remember for sure).

There was a fork of the fork, sphinx-mdinclude. We could use that project to update openapi, except that it was not compatible with recent versions of docutils, and that this would cause openapi's test suite to fail to pass.

... So we ended up forking the openapi repository to update the dependency to sphinx-mdinclude locally, and this is what we've been using since last summer. And things were good again.

But things are even better when they go upstream [citation needed]. We also filed the issue for docutils compatibility in sphinx-mdinclude. It was fixed (thanks!). We submitted a PR to have openapi switch to sphinx-mdinclude. It was adjusted (thanks!), merged, and a new tag was created.

Now at last, we can switch back to the upstream version of openapi!
And the build system lived happily ever after.

I did not run make -C Documentation update-requirements, because the resulting changes seemed to break the Netlify preview. I stuck to openapi and bumped sphinx-mdinclude to >= 0.5.2, as required by openapi.

[qmonnet]

Netlify preview breaks on the change :/.

[qmonnet]

Struggling to find the latest Netlify preview (I can't log in), but I think the latest build is here, working fine.

[joestringer]

Do we need a fresh version of the docs-builder image with this? I see that Travis is failing to build docs due to a dependency conflict.

[qmonnet]

Do we need a fresh version of the docs-builder image with this?

Yes please. It shouldn't be strictly necessary for the docs to keep building, but it would allow us to delete the cilium/openapi fork. I warned the folks who had started to rely on it, they're switching back to upstream as well.

I see that Travis is failing to build docs due to a dependency conflict.

Good catch, thanks! I don't understand why I failed to observe this either locally or in the CI workflow for docs or on the Netlify build 🤔. I bumped sphinx-mdinclude in requirements.txt, let's see how it goes now.

[joestringer]

I've triggered a new docs-builder job with these changes, tag 2023-02-04.

[qmonnet]

Thanks, I'll send a PR to update the image.

I see that the 1.12 branch is also using the fork, so I'll tag the current PR for backport there. I think it should be safe to backport, although I haven't tested.
Branches v1.11 and older did not switch to the openapi fork.