Release v2.21.0

[khaeru]

PR checklist

• Checks all ✅
• Update documentation
• Update doc/whatsnew.rst

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.15%. Comparing base (f8560ab) to head (af0fc65).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #219      +/-   ##
==========================================
- Coverage   98.88%   98.15%   -0.73%     
==========================================
  Files         101      101              
  Lines        8399     8399              
==========================================
- Hits         8305     8244      -61     
- Misses         94      155      +61     

see 18 files with indirect coverage changes