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 72.6%. Comparing base (556c088) to head (0b26ad2).
⚠️ Report is 182 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #2783      +/-   ##
===========================================
+ Coverage     55.5%    72.6%   +17.1%     
===========================================
  Files          644      660      +16     
  Lines       124135   132293    +8158     
===========================================
+ Hits         68947    96122   +27175     
+ Misses       55188    36171   -19017     

see 580 files with indirect coverage changes

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