Adds astype config for Delta in Zarr 3 wrapper

[normanrz]

Adds astype config for Delta in Zarr 3 wrapper.
Fixes #663

• Unit tests and/or doctests in docstrings
• Tests pass locally
• Changes documented in docs/release.rst
• Docs build locally
• GitHub Actions CI passes
• Test coverage to 100% (Codecov passes)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.92%. Comparing base (f43449b) to head (ff6b471).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #664   +/-   ##
=======================================
  Coverage   99.92%   99.92%           
=======================================
  Files          62       62           
  Lines        2721     2736   +15     
=======================================
+ Hits         2719     2734   +15     
  Misses          2        2           

Files with missing lines	Coverage Δ
numcodecs/tests/test_zarr3.py	100.00% <100.00%> (ø)
numcodecs/zarr3.py	100.00% <100.00%> (ø)

[rabernat]

Amazing, thanks for the quick action.