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 80.005%. Comparing base (91c76df) to head (d439147).

Additional details and impacted files

@@            Coverage Diff            @@
##           develop      #295   +/-   ##
=========================================
  Coverage   80.005%   80.005%           
=========================================
  Files           20        20           
  Lines         3546      3546           
=========================================
  Hits          2837      2837           
  Misses         709       709           

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