Drop Python 3.7 support.

[jesnie]

There's a bunch of stuff that's more difficult because of having to support the whole range of Python 3.7-3.11. Particularly around TensorFlow, mpyp and pylint. Let's drop support for some of the ancient 💩 .

[uri-granta]

(Trieste, gpflux and gpflow are still supporting Python 3.7, but hopefully we'll drop that soon.)