Fix creating groups with group()

[dstansby]

Fixes #3406, and another go at #3426 - sorry for duplicating work @d-v-b, but I thought it was easier to start again.

I decided the easiest fix here was to delegate to open_group, instead of rejigging a bunch of logic that's in open_group anyway.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.96%. Comparing base (00e7814) to head (fbce369).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3431      +/-   ##
==========================================
+ Coverage   94.92%   94.96%   +0.03%     
==========================================
  Files          79       79              
  Lines        9503     9487      -16     
==========================================
- Hits         9021     9009      -12     
+ Misses        482      478       -4     

Files with missing lines	Coverage Δ
src/zarr/api/asynchronous.py	90.87% <100.00%> (+0.84%)	⬆️

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[d-v-b]

thanks @dstansby!