Improve testing matrix

[AlexWaygood]

• Test with PyPy 3.11 as well as PyPy 3.9 and PyPy 3.10
• Remove a hack for installing cattrs on 3.13 that is no longer required

[AlexWaygood]

I considered starting to test on 3.14 for some third-party projects that have already started testing with 3.14 in CI (typeguard, mypy), but it still maybe feels a bit early -- tests might start failing for reasons unrelated to typing_extensions? Not sure though.

Similarly, every time we've tried running third-party tests on PyPy we've experienced that the third-party test suites are very flaky under PyPy, so I haven't added PyPy3.11 to any of the third-party test matrixes.

[AlexWaygood]

The cattrs test failure isn't related to this PR -- it's already failing intermittently on our main branch (#621). I opened an issue upstream at python-attrs/cattrs#669, and a proposed fix upstream at python-attrs/cattrs#670

[AlexWaygood]

The cattrs test failure isn't related to this PR -- it's already failing intermittently on our main branch (#621). I opened an issue upstream at python-attrs/cattrs#669, and a proposed fix upstream at python-attrs/cattrs#670

(Now fixed upstream; the CI job here passed on a rerun)