Bump default Python version to the latest stable (3.14)

[ulgens]

Following up #57

[ngnpope]

Previously we've kept the default version as the minimum supported version of Python on the Django main branch. This ensures that any changes to Django tested using this tool cannot introduce syntax, etc. from newer Python versions.

Granted, this is a small risk given that CI will catch this anyway, but perhaps could lead to friction for first time contributors to Django. I'm not going to suggest we change this back now, but worth being aware of.