Make all codecs pickleable

[TomNicholas]

Attempt to fix #744, using the __init_subclass__ approach suggested by @d-v-b.

I think this approach seems promising, but currently tests fail, and I don't know why. I get errors like

    def get_codec(config):
        """Obtain a codec for the given configuration.
    
        Parameters
        ----------
        config : dict-like
            Configuration object.
    
        Returns
        -------
        codec : Codec
    
        Examples
        --------
    
        >>> import numcodecs as codecs
        >>> codec = codecs.get_codec(dict(id='zlib', level=1))
        >>> codec
        Zlib(level=1)
    
        """
        config = dict(config)
        codec_id = config.pop('id', None)
        cls = codec_registry.get(codec_id)
        if cls is None and codec_id in entries:
            logger.debug("Auto loading codec '%s' from entrypoint", codec_id)
            cls = entries[codec_id].load()
            register_codec(cls, codec_id=codec_id)
        if cls:
            return cls.from_config(config)
>       raise UnknownCodecError(f"{codec_id!r}")
E       numcodecs.errors.UnknownCodecError: codec not available: ''Blosc''

TODO:

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

[TomNicholas]

for numcodecs.zarr3.Blosc this returns

Blosc(codec_name='numcodecs.Blosc', codec_config={})
numcodecs.Blosc

            See :class:`numcodecs.blosc.Blosc` for more details and parameters.
            

which all seems correct to me?

[TomNicholas]

All this can be deleted once the changes in this PR are applied to all the other codecs

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.96%. Comparing base (3438e16) to head (6d04f9b).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #745      +/-   ##
==========================================
- Coverage   99.96%   99.96%   -0.01%     
==========================================
  Files          63       63              
  Lines        2736     2708      -28     
==========================================
- Hits         2735     2707      -28     
  Misses          1        1              

Files with missing lines	Coverage Δ
numcodecs/tests/test_zarr3.py	100.00% <100.00%> (ø)
numcodecs/zarr3.py	99.39% <100.00%> (-0.11%)	⬇️

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[chuckwondo]

@TomNicholas, I believe my suggested changes should get the tests to pass.

[TomNicholas]

You legend @chuckwondo 🙇‍♂️

[TomNicholas]

pre-commit.ci autofix

[TomNicholas]

These codecs break the pattern of the ones above because they have "numcodecs.checksum32.<name>" instead of "numcodecs.<lowercasename>.<name>". I ignored that, but it doesn't seem to matter whatsoever?? Suspicious.

[TomNicholas]

@d-v-b @jakirkham this is ready for review. It would be nice to release it quickly if possible 😁

[TomNicholas]

I don't remember adding this - maybe the linter did??