Pin fastjsonschema for all pythons

[glatterf42]

For now, this is just an attempt to fix today's test failure. fastjsonschema changing from v2.20.0 to 2.21.0 was the only difference I could find to yesterday's successful run, though I'm not convinced this is the culprit.

It seems like pinning fastjsonschema actually works, but I still don't understand why, unfortunately, as it doesn't show up in the traceback.

How to review

• Read the diff and note that the CI checks all pass.
• Run a specific code snippet or command and check the output.
• View the preview build of the documentation and look at a certain page.
• Ensure that changes/additions are self-documenting, i.e. that another
  developer (someone like the reviewer) will be able to understand what the code
  does in the future.

PR checklist

• Continuous integration checks all ✅
• Add or expand tests; coverage checks both ✅
• Add, expand, or update documentation.
• Update release notes.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.9%. Comparing base (02dadfa) to head (f124d40).

Additional details and impacted files

@@          Coverage Diff          @@
##            main    #551   +/-   ##
=====================================
  Coverage   98.9%   98.9%           
=====================================
  Files         44      44           
  Lines       4790    4790           
=====================================
  Hits        4740    4740           
  Misses        50      50           

[glatterf42]

All checks are passing without any change. So did today's CI, but only after specifically re-running all jobs, not just the one that encountered the issue. We might be required to do that again in the future. I'll close this PR, then.