Include license file in the generated wheel package (celery#4404)

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

Distributing a wheel now complies with license:

> 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.

setup.cfg

@@ -22,3 +22,6 @@ requires = pytz >= 2016.7
 
 [bdist_wheel]
 universal = 1
+
+[metadata]
+license_file = LICENSE