Use license_files instead of deprecated license_file in setup

[larrybradley]

Description

setuptools has deprecated license_file in favor of license_files (pypa/setuptools#2620). Currently one will get this warning:
SetuptoolsDeprecationWarning: The license_file parameter is deprecated, use license_files instead.

license_files has been an option since 2019 (pypa/setuptools#1767).

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[pllim]

LGTM. Thanks!