Support Python 3.11

[icemac]

Look through the change log of Python 3.11 (once the final version is released) for potential issues which need to be handled by RestrictedPython to prevent access to otherwise forbidden data/objects.

There is no need to support new language features of Python 3.11 right now, let's see how they evolve when started being used.

Open tasks:

• [X] make package installable and fix tests (#223)
• [X] cut an alpha dev release (5.3a1.dev0)
• [x] drop Python 2 support (and Python 3.5) for a sane Python 3.11 support (See #223 for details)
• [ ] look through the change log of the final Python 3.11.0 version for potential changes allowing to bypass restrictions