Get rid of ZopeTestCase

[mauritsvanrees]

Every week a job on Jenkins checks which packages still use ZopeTestCase instead of plone.testing/plone.app.testing. Apart from some code in Zope itself, and some comments and backward compatibility code in plone(.app).testing, the only real use of this is in plone.schemaeditor. It is this code:

from Testing import ZopeTestCase as ztc
..
ztc.FunctionalDocFileSuite(...)

We should rewrite this to something else, probably just importing something from doctest.
We can also get rid of Py23DocChecker and any use of six, because the master branch is for Plone 6 only.

[gforcada]

Oh, so you notice that job 😄 ✨

I tried a few times, and somehow never got around to fix it 🤷🏾

We could expand that job to check for six or other stuff, though, we could do that only for packages that are Plone 6.0 only, which makes it a bit more difficult.

We could instead follow zopefoundation lead and create a meta repository to configure GitHub Actions on a per-repository basis, but keeping the source in a single specific tool.

For linting, and running tests that would be great.

We could even make these GHA cut releases whenever a tag is pushed ✨

@mauritsvanrees I'm 90% sure coming to Alpine City sprint, we can work on such things there if we haven't figured them out before 😄

[mauritsvanrees]

I like the Zope meta approach, basically one place where we keep the canonical version of most package configuration.