Build wheels for Python 3.12 as well.
Update various dependencies. The upload/download artifact actions
were using deprecated versions, and we were using a deprecated
macos build image. While we're at it, update the other OS versions
and cibuildwheel.
The main reason to drop this version before it reaches its EOL is so
that we can begin to use unittest.IsolatedAsyncioTestCase to replace
deprecated portions of the tornado.testing module.
A secondary reason is that Python 3.8 introduced support for the
samesite cookie attribute, which allows us to deprecate the
xsrf_token mechanism.
This will produce cross-version wheels so we won't have to do
releases just to make new wheels when new versions of Python
are available.
Also modernize the build process a bit to require setuptools and use
the native "optional" flag for extensions.
Our previous CI on Travis has been broken for a while due to their
move from travis-ci.org to travis-ci.com. Instead of fixing things up
there, move everything to github actions.
Ben Darnell