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

Author: jdufresne

setup.cfg

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