Fix setuptools deprecation warning about license_files

[lpsinger]

Resolves this deprecation warning:

********************************************************************************
The license_file parameter is deprecated, use license_files instead.

By 2023-Oct-30, you need to update your project and remove deprecated calls
or your builds will no longer be supported.

See https://setuptools.pypa.io/en/latest/userguide/declarative_config.html for details.
********************************************************************************

[codecov[bot]]

Codecov Report

Merging #187 (6a25e3a) into main (ce8d42e) will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #187   +/-   ##
=======================================
  Coverage   88.27%   88.27%           
=======================================
  Files           6        6           
  Lines         503      503           
=======================================
  Hits          444      444           
  Misses         59       59           

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