pin numcodecs in v3 regression tests

[d-v-b]

Pin the numcodecs version used for our v3 dtype regression tests.

TODO:

• Add unit tests and/or doctests in docstrings
• Add docstrings and API docs for any new/modified user-facing classes and functions
• New/modified features documented in docs/user-guide/*.md
• Changes documented as a new file in changes/
• GitHub Actions have all passed
• Test coverage is 100% (Codecov passes)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 61.95%. Comparing base (8eb244f) to head (edf545a).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3590      +/-   ##
==========================================
+ Coverage   59.25%   61.95%   +2.70%     
==========================================
  Files          86       86              
  Lines       10170    10170              
==========================================
+ Hits         6026     6301     +275     
+ Misses       4144     3869     -275     

see 7 files with indirect coverage changes

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[d-v-b]

the failing docs will be fixed by #3591