(chore) Type hints for abc codec `codec_id` attribute

[d-v-b]

I'm not sure how to type the buffer parameters. As long as we have a pickle codec, I don't think it's possible to restrict the return type of decode to anything narrower than object.

• [ ] Unit tests and/or doctests in docstrings
• [ ] Tests pass locally
• [ ] Docstrings and API docs for any new/modified user-facing classes and functions
• [ ] Changes documented in docs/release.rst
• [ ] Docs build locally
• [ ] GitHub Actions CI passes
• [ ] Test coverage to 100% (Codecov passes)

[d-v-b]

i'm just annotating the type of the codec_id attribute -- it's a ClassVar[str], (no default value) instead of str | None = None

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 99.96%. Comparing base (a2bdbe5) to head (c22b97f).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #702   +/-   ##
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  Coverage   99.96%   99.96%           
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  Files          63       63           
  Lines        2754     2754           
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  Hits         2753     2753           
  Misses          1        1           

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

[dstansby]

I think my suggested change (that I applied) revealed (sort of) a real issue - Checksum32 is a base class, not a codec that can be used by itself. I've made this better by making it an abstract base class in https://github.com/zarr-developers/numcodecs/pull/711 - if/when that's merged, we can update the check here in the test to not isinstance(codec, abc.ABC).