MAINT fix metadata licence

[glemaitre]

This PR makes changes to make the metadata compliant with PEP639.
In short, one should use license and license-files that then will be translated into the right metadata (cf. https://packaging.python.org/en/latest/guides/licensing-examples-and-user-scenarios/#licensing-examples-and-user-scenarios).

Some tools (like pixi) can currently failed when inspecting the metadata:

Error:  × failed to solve requirements of environment 'dev' for platform 'osx-arm64'
  ├─▶   × failed to extract metadata for package 'skada'
  │   
  ╰─▶   × Failed to parse recipe
      
      Error:
        × failed to parse SPDX license: 'COPYING', because: unknown term
        │ See <https://spdx.org/licenses> for the list of valid licenses.
        │ Use 'LicenseRef-<MyLicense>' if you are using a custom license.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.92%. Comparing base (966861a) to head (b4475b8).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #359      +/-   ##
==========================================
- Coverage   95.93%   95.92%   -0.02%     
==========================================
  Files          66       66              
  Lines        7236     7236              
==========================================
- Hits         6942     6941       -1     
- Misses        294      295       +1     

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