Cleanup crc32c soft dependency

[jakirkham]

Instead of creating a wrapper function around the call to crc32c's to check if it can be imported. Try importing crc32c when numcodecs.checksum32 is imported. If it cannot be imported, don't define the CRC32C class and skip registering it at the top-level with other Codecs.

This matches the behavior that Numcodecs has with all other optional codecs and cuts out unneeded overhead when using this checksum routine.

xref: #613

---

TODO:

• 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)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.88%. Comparing base (ad1ad67) to head (8e7e0b4).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #637   +/-   ##
=======================================
  Coverage   99.88%   99.88%           
=======================================
  Files          62       62           
  Lines        2708     2723   +15     
=======================================
+ Hits         2705     2720   +15     
  Misses          3        3           

Files with missing lines	Coverage Δ
numcodecs/__init__.py	100.00% <100.00%> (ø)
numcodecs/checksum32.py	100.00% <100.00%> (ø)
numcodecs/tests/test_checksum32.py	100.00% <100.00%> (ø)

[jakirkham]

Seeing some issues with mypy below. Flagged relevant lines that may need changes. Please let me know if there is a good way to address this

[jakirkham]

Fixed the mypy issues in recent changes

[dstansby]

Looks good - just needs a changelog entry 👍

[jakirkham]

Thanks David! 🙏

Please let me know if there is anything else

[jakirkham]

Closing and reopening for CI (specifically coverage)