Include license file in the generated wheel package

The wheel package format supports including the license file. This is
done using the [metadata] section in the setup.cfg file. For additional
information on this feature, see:

https://wheel.readthedocs.io/en/stable/index.html#including-the-license-in-the-generated-wheel-file

Helps the project comply with its own license:

> 4. Redistribution. You may reproduce and distribute copies of the Work
> or Derivative Works thereof in any medium, with or without
> modifications, and in Source or Object form, provided that You meet the
> following conditions:
>
> a. You must give any other recipients of the Work or Derivative Works
> a copy of this License; and ...