Port all doctest to python 3

[pbauer]

As discussed in #2217 and #2184 (comment) doctests are not handled when using sixer.

So we need to go through all our packages again and start migrating doctests like this

$ sixer all -w src/Products/CMFPlone/tests/*.txt

sixer does for some reason not really fix doctests, so you will have to do that by hand 😭

Getting doctest compatible with py2 and py3 can be hard though. What can help is using regex patterns to normalize the output as described in https://github.com/zopefoundation/zope.testing/blob/master/src/zope/testing/renormalizing.txt. Examples of how that is used can be found in https://github.com/zestsoftware/zest.releaser/blob/master/zest/releaser/tests/test_setup.py#L43

The best long-term option though is to port all doctests (that are not actually testing documentation) to unittests.