Upgrade supported python version

[nautik1]

I wanted to add a feature in dh-virtualenv (installing deps from a pep751 pylock.toml file, see nautik1#1 ), so I though I would first make the tests pass with a recent python.

This MR

• changes tox & travis py versions to 3.12, 3.13 and 3.14
• switches tests to pytest, removes python3-mock
• updates doc-related dependencies

Tested

• tox tests succeed with python 3.13
• pip install -r dev-requirements.txt && pytest succeeds with python 3.13
• invoke docs works, doc has not been reviewed in details but looks visually ok; added a generated LICENSE.rst file
• deb docker build works, tested with --build-arg distro=debian:trixie (resulting deb package not thoroughly tested, used in a pretty basic way)

Note: could not run this in travis though