Fix broken Python 3.9 tests

[rly]

Motivation

zarr 2.18.3 dropped support for Python 3.9 so our daily run all tests workflow is failing for Python 3.9 runs. I think a changelog entry is not needed for this change.

Checklist

• Did you update CHANGELOG.md with your changes?
• Have you checked our Contributing document?
• Have you ensured the PR clearly describes the problem and the solution?
• Is your contribution compliant with our coding style? This can be checked running ruff from the source directory.
• Have you checked to ensure that there aren't other open Pull Requests for the same change?
• Have you included the relevant issue number using "Fix #XXX" notation where XXX is the issue number? By including "Fix #XXX" you allow GitHub to close issue #XXX when the PR is merged.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 86.36%. Comparing base (5a9055d) to head (513c11b).

Additional details and impacted files

@@           Coverage Diff           @@
##              dev     #231   +/-   ##
=======================================
  Coverage   86.36%   86.36%           
=======================================
  Files           5        5           
  Lines        1188     1188           
  Branches      297      297           
=======================================
  Hits         1026     1026           
  Misses        103      103           
  Partials       59       59           

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