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 project comply with its own license:

> * Redistributions of source code must retain the above copyright notice,
>   this list of conditions and the following disclaimer.
>
> * Redistributions in binary form must reproduce the above copyright notice,
>   this list of conditions and the following disclaimer in the documentation
>   and/or other materials provided with the distribution.

Author: jdufresne

setup.cfg

@@ -1,2 +1,5 @@
 [bdist_wheel]
 universal = 1
+
+[metadata]
+license_file = LICENSE.txt