Fix store creation with latest zarr-python v3

[dstansby]

See failures at #642.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.81%. Comparing base (5b8c09c) to head (1cb02f7).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #644      +/-   ##
==========================================
+ Coverage   90.08%   99.81%   +9.73%     
==========================================
  Files          62       62              
  Lines        2712     2712              
==========================================
+ Hits         2443     2707     +264     
+ Misses        269        5     -264     

Files with missing lines	Coverage Δ
numcodecs/tests/test_zarr3.py	97.82% <100.00%> (+91.30%)	⬆️

... and 1 file with indirect coverage changes

[jakirkham]

Thank you both! 🙏