Support Python 3.13

[icemac]

Look through the change log of Python 3.13 (once the final version is released in October 2024) 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 3.13 right now, let's see how they evolve when started being used.

Open tasks:

• [x] make package installable and fix tests
• [x] cut an alpha dev release (7.0a2.dev0)
• [ ] security audit after final Python release (detailed instructions)
• [ ] Prepare for a new Python version after its final release (detailed instructions)