Add Python 3.14 support

This PR adds Python 3.14 support and follows the checklist at https://restrictedpython.readthedocs.io/en/latest/contributing/index.html#preperations-for-a-new-python-version. It is based on the work @loechel did in the type annotations PR #303 sans the type annotations because those broke support for Python 3.9. It also uses `zope.meta` to update packaging-related files.