Add jenkins lookup3 32-bit checksum

[mkitti]

Add Bob Jenkin's lookup3 from HDF5 via a Cython implementation

Fix #445

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)

[mkitti]

@rabernat Here's a cython implementation of jenkin's lookup3 32-bit checksum.

[mkitti]

I'm thinking of moving initval into the codec so that it is defined at the constructor.

Also, I'm thinking about a prepend value in case we need to add extra bytes to the beginning of the bytes being checksummed.

[codecov]

Codecov Report

Merging #446 (f579b00) into main (4f194b6) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##              main      #446    +/-   ##
==========================================
  Coverage   100.00%   100.00%            
==========================================
  Files           55        56     +1     
  Lines         2121      2238   +117     
==========================================
+ Hits          2121      2238   +117     

Impacted Files	Coverage Δ
numcodecs/__init__.py	100.00% <100.00%> (ø)
numcodecs/checksum32.py	100.00% <100.00%> (ø)
numcodecs/tests/test_jenkins.py	100.00% <100.00%> (ø)

[martindurant]

test_jenkins.py is showing lots of lint errors

[mkitti]

Could someone run CI?

[martindurant]

Missing coverage: https://app.codecov.io/gh/zarr-developers/numcodecs/pull/446/blob/numcodecs/checksum32.py

[mkitti]

Flake8 issues resolved

[mkitti]

I made some changes to docs/Makefile. When I ran docs/Makefile without having sphinx on my path, I got the following cryptic Makefile error:

$ ~/src/numcodecs/docs$ make
Makefile:12: *** recipe commences before first target.  Stop.

This was due to the if statement near the top of the file detecting the lack of sphinx, and then proceeding to try to print the "friendly" error message. However, it accomplishes exactly the opposite.

[mkitti]

I modified fletcher32 since the out keyword was not working in my hands. I also added a test for it.

[mkitti]

Let me know if you vwould prefer me to break this up a bit. I originally intended that but slipped in a commit -a by accident.

[martindurant]

Yes, please keep only the jenkins stuff in here. The other bits look OK I think, but it's better to keep PRs focussed where possible.

[mkitti]

I split off unrelated changes into #449 (Fletcher32) and #451 (sphinx Makefile)