Update Python

[puqeko]

• Adds Python 3.14 as default (replacing 3.8).
• Adds PyPy 3.11 as other (replacing 3.6).

CPython is compiled from source while binaries are fetched for PyPy.

[coveralls]

coverage: 37.654%. remained the same
when pulling 28b68cd on update-python
into db56d10 on master.

[Holmes98]

I'm not strongly opposed to this, but I'd prefer that we try to upgrade/replace the server off of xenial first, as that would make the Python upgrades much simpler and otherwise it kinda gives us an excuse to stay on xenial for longer.

Also slightly hesitant about using the pre-release version in production; probably safer to use 3.13 or to wait for the stable release of 3.14.