Use tox to run the full test suite

[kurtmckee]

Hi @corydolphin! I'm planning to use flask-cors and found that the test suite could be improved. For example, nose hasn't been updated in 8 years, and Flask 3 doesn't appear to be tested. When I ran the test suite, pytest noted a DeprecationWarning in the test suite.

This PR introduces a tox configuration. This allows flask-cors to be tested across all supported CPython and Flask versions locally (including CPython 3.12, which is now listed as a supported version).

This work also updates CI to run the tests using tox.

Please let me know if you have questions about these changes.

[kurtmckee]

@corydolphin I've discovered that the documentation isn't getting built on Read the Docs (recent build failures).

If you can review this work and are comfortable merging it, I can build off this work to both test the documentation builds using tox and resolve the failures in Read the Docs.

Let me know either way; I'd like to cut a PR to fix the documentation builds ASAP. Thanks!

[kurtmckee]

I've addressed the failing documentation builds separately, but note that testing doc builds -- and updating documentation build dependencies -- will be significantly easier using tox.