DEBUG (not for merging): issue 4420 reproducer

[tylerjereddy]

The development team requested (scipy/scipy#17644 (comment)) a reproducer in the CI for gh-4420.

• first commit cuts down the CI to prevent wasting
• second commit formalizes some of my reproducing instructions so you can see the failure with the latest release of pybind11 in CI (if you switch down to the 2.10.1 release there you should see the job pass)
• I realise you might prefer to build master from source to debug rather than me pip installing the latest stable release in the repo proper, but obviously this team should be able to alter this workflow to do that if desired..

[tylerjereddy]

You can see the reproducing on my fork here: tylerjereddy#1

You may not see it in this PR without approving the CI to run for 1st time contributor.

[rwgk]

Awesome, thank you! I see the error reproduces in the workflow. That's a great start.