Include license file in the generated wheel package (#484)

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 project comply with its own license:

> The above copyright notice and this permission notice shall be
> included in all copies or substantial portions of the Software.