fix pcodec resolution failure

[d-v-b]

fixes a typo leading to failure to register pcodec properly in the config.

this PR adds pcodec and zfpy to our main test dependencies. we should consider splitting these codecs out into their own dependency group if necessary.

I also fix some tests that were not running properly, and adapt some docstring tests to the new docs.

closes #3482

[rabernat]

Great

[d-v-b]

I'm removing the pcodec test dependency because we do tests against an older version of numcodecs that used a different (incompatible) PCodec codec.

[codecov]

Codecov Report

❌ Patch coverage is 0% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 61.21%. Comparing base (0278494) to head (7dbb633).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/zarr/codecs/__init__.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3483   +/-   ##
=======================================
  Coverage   61.21%   61.21%           
=======================================
  Files          84       84           
  Lines        9923     9923           
=======================================
  Hits         6074     6074           
  Misses       3849     3849           

Files with missing lines	Coverage Δ
src/zarr/codecs/__init__.py	0.00% <0.00%> (ø)

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