Add support for Python 3.12 and drop EOL 3.7

[hugovk]

The Python 3.12 release candidate is out! 🚀

Call to action

We strongly encourage maintainers of third-party Python projects to prepare their projects for 3.12 compatibilities during this phase, and where necessary publish Python 3.12 wheels on PyPI to be ready for the final release of 3.12.0.

See also https://dev.to/hugovk/help-test-python-312-beta-1508/

---

Python 3.7 is EOL and no longer receiving security updates (or any updates) from the core Python team.

version	released	eol
3.11	2022-10-24	2027-10-24
3.10	2021-10-04	2026-10-04
3.9	2020-10-05	2025-10-05
3.8	2019-10-14	2024-10-14
3.7	2018-06-26	2023-06-27

https://devguide.python.org/versions/

[regebro]

I was in two minds about changing the minimum python version, as 3.7 still technically works, but then I remembered that anyone still on Python 3.7 isn't likely to upgrade packages either. :-D