Migrate from setup.py + setuptools to pyproject.toml + hatchling

[hugovk]

Fixes #151.

Replaces the deprecated license classifiers with PEP 639 metadata: https://hugovk.dev/blog/2025/improving-licence-metadata/

[hugovk]

TypeError: Field `project.license-files` must be a table

The new metadata isn't supported for 3.6 and 3.7: https://github.com/hugovk/wcwidth/actions/runs/19575036750/job/56057960860

Let's drop them, they've been EOL since 2021-12-23 and 2023-06-27:

https://devguide.python.org/versions/

[GalaxySnail]

By thw way, since setuptools >= 61 supports PEP 621, I think we can use pyproject.toml + setuptools.

[hugovk]

Any particular reason to stick with setuptools instead of something more modern?

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (a753ba4) to head (e927f5d).
⚠️ Report is 13 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##            master      #156   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            6         6           
  Lines          100       100           
  Branches        20        20           
=========================================
  Hits           100       100           

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[GalaxySnail]

One reason is that we can keep the setup.py in this version, which generates a warning for those who are still calling python setup.py. And since setuptools works well with modern specifications, there's no need to replace it.