Support testing against Python 3.13

[rmartin16]

Changes

• Add support for CI to test Python 3.13
• Add support for tox to run using Python 3.13

Issues

• coverage needs support for Python 3.13 (added with v7.4.1)
• Need a pytest release that includes Using Python 3.13.0a3+ on Windows when USERNAME env var is masked/unset, get_user() leaks OSError and breaks (at least) tmp_path usage pytest-dev/pytest#11874

PR Checklist:

• All new features have been tested
• All new features have been documented
• I have read the CONTRIBUTING.md file
• I will abide by the code of conduct

[freakboy3742]

Content of the PR looks good; although it's missing the setup.cfg metadata declaring Py3.13 compatibility.

On top of that, I think we should hold off merging until coverage and CI is passing. We have a higher threshold for failures on dev releases, but as we're still early in the 3.13 cycle, I think we can afford to hold off until there's basic support in place.

[rmartin16]

Agreed; I was mostly just curious where support stands. At a minimum, we're waiting on a release of coverage.

[rmartin16]

Pytest 8.0.1 released with the needed fix. I also added the classifier for Python 3.13.

[freakboy3742]

Looks fairly straightforward; Approved on the basis that it does what it says on the tin.

As a follow up to this - we should probably turn on macOS ARM64 testing as well. That will start to explode the size of the CI matrix to absurd levels - but we can probably cut back on doing every python version on every OS, and limit ourselves to "min and max versions" (or min, max and dev) on every OS, and every version on one OS (probably linux, given there are more available runners).