Update in setup.cfg: license-file -> license_files

[martenwa]

This happens both in version 0.5.0 and in master:

 * QA Notice: setuptools warnings detected:
 * 
 * 	The license_file parameter is deprecated, use license_files instead.
 * 	Usage of dash-separated 'license-file' will not be supported in future versions. Please use the underscore name 'license_file' instead

[dbieber]

Sounds good. Let's do the rename.

[hugovk]

In fact, it can be omitted entirely because setuptools automatically includes LICENSE:

A list of glob patterns for license related files that should be included. If neither license_file nor license_files is specified, this option defaults to LICEN[CS]E*, COPYING*, NOTICE*, and AUTHORS*.

https://setuptools.pypa.io/en/latest/references/keywords.html#keywords

Please see PR #434.

[dbieber]

Closed by #434