Enable Python 3.14

[nphilipp]

Story

As an operator of Duffy,
I want that it runs on Python 3.14,
so I can run it on platforms using it.

Acceptance Criteria

• The test suite runs successfully when run under Python 3.14 (e.g. with tox -e py314
• CI is configured to run the test suite with Python 3.14.

[nphilipp]

This doesn’t work yet. Version 2.11.7 (the current stable version) of pydantic requires pydantic-core==2.33.2 (which doesn’t build on Python 3.14). The 2.12.0a1 pre-release of pydantic requires pydantic-core==2.37.2 and it installs cleanly ⇒ Python 3.14 most likely needs pydantic>=2.12 (once its available) and pydantic-core>=2.37.2 (which is available, but pydantic pins specific pydantic-core versions.