Handle extra non-Enthought copyright line(s)

[corranwebster]

There are a number of files in ETS, particularly handling Qt support which also have copyright attributed to Riverbank Computing, for example https://github.com/enthought/pyface/blob/master/pyface/ui/qt4/action/action_item.py which starts with:

# (C) Copyright 2007 Riverbank Computing Limited
# (C) Copyright 2005-2020 Enthought, Inc., Austin, TX
# All rights reserved.
#
# This software is provided without warranty under the terms of the BSD
# license included in LICENSE.txt and may be redistributed only under
# the conditions described in the aforementioned license. The license
# is also available online at http://www.enthought.com/licenses/BSD.txt
#
# Thanks for using Enthought open source!

These are sufficiently common it would be good to just accept this variant. The simplest way to do this would be to add the line as an optional part of the regular expression, but it may make sense to make something a bit more general.