[CI] list the LICENSE as a license_file

[emasab]

Data files are installed in the wrong directory when packaging this, and it is very uncommon to do so. data files should be files needed for the runtime only.

[emasab]

Thanks @dirkmueller @asottile-sentry ! Chose @dirkmueller one because license_files is available since setuptools 42 (Nov 2019) and it's older than 3 years. Confluent Platinum end of support

[emasab]

See #713 #1336.

[asottile-sentry]

why not just merge one of the other PRs instead of committing someone else's PR as yourself?

[emasab]

@asottile-sentry the author is still @dirkmueller. We cannot merge directly because our CI won't run on external code and that code could change any moment, so for security reasons. We're still acknowledging contributions in CHANGELOG.

[asottile-sentry]

talk about "open source" only in name

[emasab]

We're working on a way to run CI directly on external PRs, still after manual review of its code, so we don't have to do this.