Include license file in the generated wheel package (#243)

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

setup.cfg

@@ -11,5 +11,8 @@ create-wheel = yes
 [bdist_wheel]
 universal = 1
 
+[metadata]
+license_file = LICENSE.rst
+
 [tool:pytest]
 norecursedirs = .* env* docs *.egg src/icalendar/tests/hypothesis