Remove a mypy error message workaround that should be unnecessary in mypy 1.15.0

[wyattscarpenter]

See https://github.com/python/mypy/pull/17485 or https://github.com/python/mypy/issues/10768 for more information on the problem and one-net-line workaround that used to be here.

This removes a workaround for the problem addressed by "Fix --install-types masking failure details (wyattscarpenter, PR 17485)" mentioned in https://mypy.readthedocs.io/en/stable/changelog.html , which I had previously added a workaround for in the course of contributing a prior PR. (It's fine if the workaround remains in; it doesn't break anything (I think) — although future maintainers may find it a bit confusing!)

This PR drops python 3.8 support, because the fixed version of mypy drops 3.8 support, and so is only useful after this project drops 3.8 support. (Maybe that's already happened.)

While I was here, I also changed something to "poetry.group.dev.dependencies", as the poetry deprecation message suggested, which didn't seem to change anything.

I also ran poetry lock to update the lock file, which was needed.

The DCO check is currently broken, but I have signed-off all of my commits here 🙇

Appendix A: verbiage from when this was a draft PR

This pr should be good to go next time there's another release of mypy. I'm just keeping it a draft til then.

(I also think I will have to run poetry lock, based on the ci. But I'lll put that off until mypy 1.15.0 is even real.)