Cleanup PCodec soft dependency handling

[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 (86f5ce4) to head (4cc252e).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #647      +/-   ##
==========================================
- Coverage   99.92%   99.85%   -0.08%     
==========================================
  Files          62       62              
  Lines        2721     2720       -1     
==========================================
- Hits         2719     2716       -3     
- Misses          2        4       +2     

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

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• Flaky Tests Detection - Detect and resolve failed and flaky tests

[jakirkham]

@dstansby could you please review this one?