Migrate build system from setuptools to meson-python

[maxrjones]

This PR includes a complete modernization of the build setup for numcodecs. I could break these changes into individual, more easily reviewable PRs, but I wanted to first open a proof of concept with the final state to gauge appetite for this scale of change. The motivation here is to reduce the maintenance burden for numcodecs. I'd like to enable small fixes or releases without developers needing to spend hours futzing with the build/ci setup, which is unfortunately how I spent my afternoon/evening.

Concrete changes included in this modernization:

• Replace setup.py with meson.build for all 8 C/Cython extensions
• Move to src/ layout with top-level tests/
• Add system library support (-Dsystem_blosc, -Dsystem_zstd, -Dsystem_lz4)
• Fix cross-compilation SIMD detection (meson host_machine vs py-cpuinfo)
• Update CI workflows and cibuildwheel config
• Update contributing guide to recommend uv/venv

I tested these changes locally, but will need to check if there's any issues with the ci.

Closes #509
Closes #824
Closes #703

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 100.00%. Comparing base (06a53c6) to head (a1c9a43).

Additional details and impacted files

@@             Coverage Diff             @@
##             main      #826      +/-   ##
===========================================
+ Coverage   99.33%   100.00%   +0.66%     
===========================================
  Files          26        27       +1     
  Lines         898       902       +4     
===========================================
+ Hits          892       902      +10     
+ Misses          6         0       -6     

Files with missing lines	Coverage Δ
src/numcodecs/__init__.py	100.00% <100.00%> (ø)
src/numcodecs/abc.py	100.00% <ø> (ø)
src/numcodecs/astype.py	100.00% <ø> (ø)
src/numcodecs/base64.py	100.00% <ø> (ø)
src/numcodecs/bitround.py	100.00% <ø> (ø)
src/numcodecs/bz2.py	100.00% <ø> (ø)
src/numcodecs/categorize.py	100.00% <ø> (ø)
src/numcodecs/checksum32.py	100.00% <ø> (ø)
src/numcodecs/compat.py	100.00% <ø> (ø)
src/numcodecs/delta.py	100.00% <ø> (ø)
... and 17 more

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[d-v-b]

thanks so much for picking this up max!

[d-v-b]

one possibility to put on the table: we could consider putting codecs with orthogonal dependencies in separate sub-projects, and make this project a monorepo. We should only think about this if it makes things simpler of course.

[maxrjones]

Thanks so much for your prompt review @QuLogic! I believe that I addressed all your helpful comments in the latest set of commits.

[maxrjones]

one possibility to put on the table: we could consider putting codecs with orthogonal dependencies in separate sub-projects, and make this project a monorepo. We should only think about this if it makes things simpler of course.

I think this could be a separate task. I'm a bit ambivalent about that change, but am really motivated by this PR because it'll help us keep all the checks green.

[d-v-b]

one possibility to put on the table: we could consider putting codecs with orthogonal dependencies in separate sub-projects, and make this project a monorepo. We should only think about this if it makes things simpler of course.

I think this could be a separate task. I'm a bit ambivalent about that change, but am really motivated by this PR because it'll help us keep all the checks green.

makes sense! this PR looks great as-is