Delay initialization of multiprocessing.Lock until it's needed

[neutrinoceros]

[Description of PR]

Fixes #522
I'm not sure how to properly test this though, as the test would need to run before numcodecs is even imported the first time, which seems hard to control with how pytest collects all tests before running any particular one.

TODO:

• Unit tests and/or doctests in docstrings
• Tests pass locally
• [N/A] 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 99.91%. Comparing base (e3299b8) to head (45cbf87).
Report is 56 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #615   +/-   ##
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  Coverage   99.91%   99.91%           
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  Files          59       59           
  Lines        2405     2405           
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  Hits         2403     2403           
  Misses          2        2           

[dstansby]

Thanks a lot!

[jakirkham]

Thank you for fixing this long standing bug! 🙏

Would be curious to hear how you arrived at this solution?

[neutrinoceros]

I picked up a suggestion from astropy/astropy#16351 (comment), so credit goes to @saimn for figuring it out. I'm just the muscle here 😅