Support Python 3.15

[dataflake]

Look through the change log of Python 3.15 (once the first release candidate version is released end of July 2026 (see https://peps.python.org/pep-0790/) 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 3.15 right now, let's see how they evolve after they can be used,

Open tasks:

• make package installable and fix tests
• Change log: Allow to use the package with Python 3.15 -- Caution: No security audit has been done so far.
• cut an alpha dev release
• security audit after rc1 Python release (detailed instructions)
• Prepare for a new Python version after its rc1 release (detailed instructions)
• Create a copy of this ticket for Python 3.16 (first alpha is expected to land late 2026)