[CI] Use Ray fork of pytest-sphinx

[bveeramani]

Why are these changes needed?

pytest-sphinx lacks:

• Support for MyST files
• The ability to mock outputs for illustrative purposes

To address these issues, I've forked pytest-sphinx to https://github.com/ray-project/pytest-sphinx and implemented the features.

Related issue number

Checks

• I've signed off every commit(by using the -s flag, i.e., git commit -s) in this PR.
• I've run scripts/format.sh to lint the changes in this PR.
• I've included any doc changes needed for https://docs.ray.io/en/master/.
  • I've added any new APIs to the API Reference. For example, if I added a
    method in Tune, I've added it in doc/source/tune/api/ under the
    corresponding .rst file.
• I've made sure the tests are passing. Note that there might be a few flaky tests, see the recent failures at https://flakey-tests.ray.io/
• Testing Strategy
  • Unit tests
  • Release tests
  • This PR is not tested :(

[can-anyscale]

I assume we don't want to maintain pytest-sphinx. Do we have plan to merge these features to the main pytest-sphinx sometimes soon?

[bveeramani]

I assume we don't want to maintain pytest-sphinx. Do we have plan to merge these features to the main pytest-sphinx sometimes soon?

Yeah, I have a PR open: twmr/pytest-sphinx#54.

Also, the changes are minor (see ray-project/pytest-sphinx#1 and ray-project/pytest-sphinx#2), so the maintenance burden is low anyway.

[can-anyscale]

Sound good, can we add a TODO or an issue somewhere to follow up and move back to the original pytest-sphinx.