Add support for runtime Python version 3.12

[mhsmith]

Move checklist from https://repo.flexpansion.com/issues/5400 into the repository (and see also #932).

We will drop 32-bit support in this version (#709).

[mhsmith]

Created checklist in target/README.md.

One test is currently marked as an expected failure, which will be fixed by Python 3.12.1 (#1024).