CI: Display pytest failures as GitHub annotations in GitHub Actions

[echoix]

To help solving pytest failures currently blocking the required checks, and for general usability (without having to fully open in a IDE to navigate PRs), I suggest here to add a pytest plugin (https://github.com/pytest-dev/pytest-github-actions-annotate-failures) that bridges the pytest failures and special log format that is understood by GitHub actions (https://docs.github.com/en/actions/using-workflows/workflow-commands-for-github-actions).

They include a screenshot of what it could look like,

But here is what it looks like now, for our repo:

1. In the log, there are additional coloured lines, but that is only a side effect
2. In a PR's check tab, we see in each collapsible section an indicator when there are some annotations
   a. More specifically, this
3. When clicking on the check, the annotation count marker, or going to the summary tab of a GitHub Action workflow run, there is a list of annotations, errors only here (other tools can create warnings, notices and debug messages if wanted),
4. Clicking on an annotation brings to the code, with the line in question, here all are line 1 (why?)
5. I created a failure in another test run by pytest in another of my branches to illustrate another line:
   a.
   b. When the error is in a modified file:

[wenzeslaus]

I'm for merging this as is. It seems faster to navigate and overall helpful especially since the alternative is going through logs.

The wrong line number for Pylint warning is slightly misleading, but the warning may be potentially still more readable for some people than just the log. It does not happen always. I wonder if the original in the log is better. That seems like a good fallback.

[echoix]

I think it's the plugin that doesn't report the line number like a real pytest test.