chore: improve license metadata (PEP 639)

[mwtoews]

Improve license metadata, following PEP 639, with these changes:

• Change licence from a deprecated table structure to a simpler text field using a valid identifier: CC0-1.0
• Specify the licence file
• Remove the licence classifier, which is deprecated

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 72.8%. Comparing base (556c088) to head (7efe828).
⚠️ Report is 194 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #2783      +/-   ##
===========================================
+ Coverage     55.5%    72.8%   +17.2%     
===========================================
  Files          644      662      +18     
  Lines       124135   133024    +8889     
===========================================
+ Hits         68947    96845   +27898     
+ Misses       55188    36179   -19009     

see 583 files with indirect coverage changes

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