The Plone team is working on a migration of the CMS to Python 3. I read about this migration and that there are good progress. Thus I decided to try the ported version out. There is a repository with a working on Github. I checked it out from https://github.com/collective/demo.plone.de. I followed the instructions in the file https://github.com/collective/demo.plone.de/blob/master/local_demo_latest_py3.cfg and it worked out. I could create a new site with Plone 5.2a1 and Python 3.6.5 (see screenshot below).