build: Use license_files in setup.cfg

[matthewfeickert]

Description

Use license_files over license_file in setup.cfg to avoid

SetuptoolsDeprecationWarning: The license_file parameter is deprecated, use license_files instead.

setuptools added support for license_files in v42.0.0.

Checklist Before Requesting Reviewer

• Tests are passing
• "WIP" removed from the title of the pull request
• Selected an Assignee for the PR to be responsible for the log summary

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For the PR Assignees:

• Summarize commit messages into a comprehensive review of the PR

* Use 'license_files' over 'license_file' in setup.cfg to avoid
'SetuptoolsDeprecationWarning: The license_file parameter is deprecated, use
license_files instead.' setuptools added support for 'license_files' in v42.0.0.

[matthewfeickert]

Once I understand pypa/build#482 better I'll also add a check to the CI workflow so that these get caught automatically.

[codecov]

Codecov Report

Merging #1883 (390f6b9) into master (ebcc964) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1883   +/-   ##
=======================================
  Coverage   98.17%   98.17%           
=======================================
  Files          68       68           
  Lines        4322     4322           
  Branches      726      726           
=======================================
  Hits         4243     4243           
  Misses         46       46           
  Partials       33       33           

Flag	Coverage Δ
contrib	26.39% <ø> (ø)
doctest	60.73% <ø> (ø)
unittests-3.10	96.04% <ø> (ø)
unittests-3.7	96.02% <ø> (ø)

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[matthewfeickert]

As other maintainers are on holiday, I'm going to approve and merge this myself. As always, PRs approved by a single core dev can be reverted as needed by the rest of the dev team.