build: add py3.12, remove py3.8; use Debian 12

[uniqueg]

• [ ] Add support for py3.12
• [ ] Remove support for py3.8 (end of life: 2024-10)
• [ ] Use Python images built based on Debian Bookworm (v12) instead of Debian Bullseye (v11; end of life: 2024-07)
• [ ] Use type hint generics dict and list etc. instead of typing.Dict and typing.List etc. across the entire codebase, as this is supported from Python 3.9 onwards; see PEP 585 for details
• [ ] Check if any other changes introduced in Python 3.9 can be usefully applied to the codebase