Tell codespell to ignore "assertIn"

[rly]

Motivation

The latest version of codespell causes 10+ new check failures because it introduced the check/correction assertIn ==> asserting, assert in, assertion. We use assertIn in tests.
This PR tells codespell to ignore the word "assertIn".

See codespell-project/codespell#3430

This is super minor and I think it does not merit a changelog entry.

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 flake8 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]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.09%. Comparing base (2f851df) to head (ed41064).
Report is 17 commits behind head on dev.

Additional details and impacted files

@@           Coverage Diff           @@
##              dev    #1909   +/-   ##
=======================================
  Coverage   92.09%   92.09%           
=======================================
  Files          27       27           
  Lines        2643     2643           
  Branches      689      689           
=======================================
  Hits         2434     2434           
  Misses        139      139           
  Partials       70       70           

Flag	Coverage Δ
integration	73.13% <ø> (ø)
unit	84.07% <ø> (ø)

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[rly]

Some tests are failing because of unrelated issues related to the Zarr tutorial.