Serialize Nones

[flying-sheep]

• Fixes .write does not save None values #673
• Added tests
• Added release notes

Rendered release notes

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 84.53%. Comparing base (8e9eb88) to head (2d2ca32).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #999      +/-   ##
==========================================
- Coverage   86.99%   84.53%   -2.46%     
==========================================
  Files          40       40              
  Lines        6045     6054       +9     
==========================================
- Hits         5259     5118     -141     
- Misses        786      936     +150     

Files with missing lines	Coverage Δ
src/anndata/_io/specs/methods.py	88.93% <100.00%> (-0.17%)	⬇️
src/anndata/_io/specs/registry.py	95.40% <ø> (-0.63%)	⬇️

... and 6 files with indirect coverage changes

[ivirshup]

As discussed, I don't want to change the on-disk format in the next release. So I'd hold off on this until at least 0.11.

[flying-sheep]

Since 0.10 is released, we can document and then merge this, right?

[ivirshup]

I would like to have proposals for all disk format changes written up and discussed with other implementers before merging them to main.

[flying-sheep]

Sure, we can use this PR as reference implementation.

Which other implementers are there? Is there a channel with the relevant people on some communication medium?

[alam-shahul]

Hi there, I feel that this is an important feature/bug fix, I was wondering if there were plans to merge this anytime soon/if it's possible to help.