Update testing dependencies and fix tests for mypy 1.20

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Please note that the target branch is dev-mypy instead of main. This will likely be the last PR on dev-mypy, after this is merged I'll merge dev-mypy to main.

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mypy introduced some changes in version 1.20 which causes error messages (and reveal_type output) to look different than before. Most of this is just stripping the builtins. from builtin types (e.g. int instead of builtins.int), which is a change I quite like because it improves readability. But it also means I had to update a lot in the typing tests. 😅

This PR updates all testing dependencies to their latest versions (including mypy 1.20.0) and fixes anything that broke because of that.

Since the mypy update broke our pipelines, I've also decided to pin the versions of all testing dependencies to minor versions instead of major versions (it used to be mypy ~= 1.19 which is equivalent to mypy >= 1.19, < 2.0, now we specify still a minimum version but only allow patch level updates).

I also did some smaller changes in the test environment (e.g. modernizing the tox config a little bit, fixed some deprecated config options), and updated the GitHub Actions versions.

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Note: "post-release" has been deprecated for a while now, "no-guess-dev" is the recommended replacement. It doesn't affect normal builds, only local development builds.

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Note: This was removed because license classifiers are deprecated. We still set license = MIT in the metadata above.