Update supported versions: add newer CPython and PyPy, drop 3.7

[itamarst]

Fixes #111
Fixes #67

• Switch to modern Python 3 buffer APIs, since the old style was dropped in Python 3.13
• Drop Python 3.7 support
• Add Python 3.13 support
• Add Python 3.12 testing in place where it was missed
• Add PyPy 3.10

Note to reviewer: Reviewing the C code and CPython API usage (PyBuffer_Release etc) for correctness is extremely important, so we don't have memory unsafety issues.

[sajith]

GitHub says that macos-14 will use M1 macs, if that is useful.

https://github.blog/changelog/2024-01-30-github-actions-introducing-the-new-m1-macos-runner-available-to-open-source/

[itamarst]

Ah, that's good to know... assuming we can build a wheel that works on older macOS. Which cibuildwheel might automate. Or not.

[meejah]

LGTM!

I'm not super-familiar with the new C buffer protocol, but triple-checked the ref-counting and that at least looks good to me.