I created a further buildout for Plone from the Plone coredev Github repository and checked out the 5.2 branch. The buildout went fine and I could create a new Plone site.
I tried out to create a new add-on with bobtemplates and it worked with my new Plone site (on Python 3.6). I was able to install the add-on without errors. I’ll try out some further add-ons with this buildout using my own extension of the core buildout script. I’m curious, if this add-ons will work with my buildout. If they will not run on Python 3 I’ll try to upgrade them.