Need new tests for modern Plone and Python 3

[abosio]

I've pushed a python3 branch because we are working on an Poi update to Plone 5.2 on Python 3 for a client. I've removed the old PloneTestCase / Doctests which are broken in Python 3. Others may be able to make use of this branch and/or contribute new tests so we eventually get this merged to master.