python-icalendar changing license to BSD

[Stanislav Ochotnicky]

Upstream of python-icalendar changed to Plone foundation and they
relicensed the project from mostly LGPLv2 to BSD.

Original license:
# test.py is GPLv2
# parser is GPL
# doctest.py is Public Domain
# rest is LGPLv2
License:        LGPLv2 and GPLv2 and GPLv2+ and Public Domain

Package has been cleaned so it's no longer such a mix, and upstream went
through contributors to change the license:
https://github.com/collective/icalendar/issues/2