stack depends on Python 3.12

[dboreham]

Just won't run (throws an exception like below) on earlier Pythons. Particularly relevant because Ubuntu 22 ships with Python 3.10.
Readme should be updated to say that stack requires Python 3.12 and needs the Python in Ubuntu 22+.

"/home/bpi/.shiv/stack_2fc2bd11423f4d4fb7b4cacbb26686ee395ca1211f583b4630fd3ad41e89ca04/site-packages/stack/main.py", line 24, in <module>
    from stack.build import build, prepare
  File "/home/bpi/.shiv/stack_2fc2bd11423f4d4fb7b4cacbb26686ee395ca1211f583b4630fd3ad41e89ca04/site-packages/stack/build/build.py", line 18, in <module>
    from stack.build.build_containers import command as build_containers
  File "/home/bpi/.shiv/stack_2fc2bd11423f4d4fb7b4cacbb26686ee395ca1211f583b4630fd3ad41e89ca04/site-packages/stack/build/build_containers.py", line 179
    log_info(f"Found {len(all_containers_in_scope)} containers in {len(required_stacks)} stacks: {", ".join([c.name for c in all_containers_in_scope])}", bold=True)
                                                                                                   ^
SyntaxError: f-string: expecting '}'

[telackey]

I fixed the error so that it works on 22.04 again, but I also updated the readme to mention Python 3.12 and Ubuntu 24.

[telackey]

#78