Test and build Python 3.12, drop Python 3.7 support

[tiptenbrink]

#94 asks for wheels to be built for Python 3.12. This simply updates the CI and bumps the version to 0.6.1 so a new release can be made that includes the new Python 3.12 wheels. All tests are passing.

Probably it would be a good idea to drop Python 3.5 to 3.7 support, so if desirable I could update that as well. There's also some old actions being used, I can also bump those.

[tiptenbrink]

Pinging @fantix. Also thanks for updating other packages already (i.e. uvloop), I know it can be quite a lot of work, but here it seems manageable so that's why I'm contributing this.

[fantix]

Thanks for the PR! We usually bump the version in a separate PR/commit with changelogs (for the release bot to do its job), so let's keep this PR bumping Python versions and stuff.

Let's also fix the following to match our minimum requirement (Python 3.8):

httptools/setup.py

Line 4 in 1599df9

if vi < (3, 5):

httptools/setup.py

Line 188 in 1599df9

python_requires='>=3.5.0',

[tiptenbrink]

Alright, should be fixed now!