Add support for Python 3.15

[mwichmann]

This change affects only the ActionTests.py test, which has version-indexed bytecode strings. This is tested working as of 3.15.0a8, which was the first time the bytecode changed - and billed as the last planned alpha (now looks like there might be one more)

Note the CI won't touch the 3.15 code path... it would need a test that included something like

steps:
- uses: actions/checkout@v6
- uses: actions/setup-python@v6
  with:
    python-version: '3.15-dev'

Contributor Checklist:

• I have created a new test or updated the unit tests to cover the new/changed functionality.
• I have updated CHANGES.txt and RELEASE.txt (and read the README.rst).
• I have updated the appropriate documentation