Update license* metadata and include third-party licenses

[jakirkham]

Fixes #724

Makes a few fixes to license metadata and packaging:

• Switch to using SPDX metadata for license
• Migrate to project.license-files for packaging license files
• Include 3rd party license files
• Drop deprecated trove-classifier for license

TODO:

• Unit tests and/or doctests in docstrings
• Tests pass locally
• Docstrings and API docs for any new/modified user-facing classes and functions
• Changes documented in docs/release.rst
• Docs build locally
• GitHub Actions CI passes
• Test coverage to 100% (Codecov passes)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.96%. Comparing base (169db1b) to head (a26e72d).
Report is 12 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #729   +/-   ##
=======================================
  Coverage   99.96%   99.96%           
=======================================
  Files          63       63           
  Lines        2738     2738           
=======================================
  Hits         2737     2737           
  Misses          1        1           

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[jakirkham]

Thank you both! 🙏