Cleanup PCodec soft dependency

[jakirkham]

As with other soft dependencies in Numcodecs, the pattern is to attempt to import them and if their dependencies cannot be resolved skip them. Have applied this pattern to PCodec to be consistent with the other optional codec dependencies.

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 99.85%. Comparing base (4b95d66) to head (6dd00c0).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #647      +/-   ##
==========================================
- Coverage   99.92%   99.85%   -0.08%     
==========================================
  Files          62       62              
  Lines        2761     2759       -2     
==========================================
- Hits         2759     2755       -4     
- Misses          2        4       +2     

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

... and 1 file with indirect coverage changes

[jakirkham]

@dstansby could you please review this one?

[dstansby]

LGTM 👍 - do you want to add a changelog if this changes behaviour for users?

[jakirkham]

Resolved conflicts and added a changelog entry

[dstansby]

Suggested change

	* Cleanup ``PCodec`` soft dependency.
	* Cleanup ``PCodec`` soft dependency.
	Previously importing ``numcodecs.pcodec`` would work if ``pcodec`` is not installed,
	but now it will fail to import. This mirrors the behaviour of other optional dependencies.

[dstansby]

Would be good to have something that describes what's changed to users - I've left a suggesiton above