Remove proposed/3.0.0 branch, only ever run against Python sans matrix option

Add official Python 3.8 support on Appveyor

Apply the Apache-2.0 license correctly

This follows instructions from the Apache Foundation. It also ensures
that the license will be correctly detected by license scanning tools.

Note that by adding a NOTICE file triggers Section 4.d of the license,
which is usually intended by users of the license.

The text that was in the LICENSE file before this change is intended
(by the authors of the license) to be used as a file header, and not as
a full license notice for a project, but it is commonly used this way,
so I would of course understand if this change is rejected.

Update misleading text about python support