Enable install & testing against Python 3.12

[neiljp]

What does this PR do, and why?

This PR is intended to

• check that dependencies work satisfactorily under Python 3.12
• ensure that we install under Python 3.12

These also enabled confirmation that after the 0.2.0/0.2.1 release yanks, that installing under an unsupported python does not install a very old option any more 👍

NOTE: This does not guarantee that there are no issues with python 3.12. For that we likely need urwid 2.2+ (http://urwid.org/changelog.html#release-2-2-0), however that also removes python 3.7 compatibility.

Outstanding aspect(s)

• Wait for Python 3.12 release
• Ensure that dependencies install OK
• Tests run OK against the release?
• Manual testing?

External discussion & connections

• Discussed in #zulip-terminal in topic
• Fully fixes #
• Partially fixes issue #
• Builds upon previous unmerged work in PR #
• Is a follow-up to work in PR #
• Requires merge of PR #
• Merge will enable work on #

How did you test this?

• Manually - Behavioral changes
• Manually - Visual changes
• Adapting existing automated tests
• Adding automated tests for new behavior (or missing tests)
• Existing automated tests should already cover this (only a refactor of tested code)

Self-review checklist for each commit

• It is a minimal coherent idea
• It has a commit summary following the documented style (title & body)
• It has a commit summary describing the motivation and reasoning for the change
• It individually passes linting and tests
• It contains test additions for any new behavior
• It flows clearly from a previous branch commit, and/or prepares for the next commit