deprecate python 3.8, add testing of python 3.12

[philvarner]

Related Issue(s):

n/a

Description:

1. Restrict to require Python >=3.9 (previously unrestricted)
2. Test Python 3.12 in CI

PR checklist:

• [X] Code is formatted (run scripts/format).
• [X] Code lints properly (run scripts/lint).
• [X] Tests pass (run scripts/test).
• [X] Documentation has been updated to reflect changes, if applicable.
• [X] Examples have been updated to reflect changes, if applicable
• [X] Changes are added to the CHANGELOG.

[philvarner]

Putting this back into draft mode as the latest aiohttp release doesn't build correctly on 3.12.

aiohttp can either (1) be pinned to aiohttp==3.9.0b0 or (2) AIOHTTP_NO_EXTENSIONS=1 set for the install (per https://github.com/aio-libs/aiohttp/issues/7229)