MAINT: update testing against sphinx6, drop sphinx3

[mmcky]

This PR updates the testing infrastructure to test against sphinx4 and sphinx5 and sphinx6 (dropping sphinx3).

It also replaces sphinx-panels with sphinx-design as sphinx-panels is now deprecated.

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[mmcky]

• [x] this is dependent on sphinx-book-theme which is currently set to sphinx<5 so it is currently testing sphinx==4.5. This PR has been tested on sphinx5

[choldgraf]

Happy to review a PR over in the book theme to bump it up to Sphinx 5 compatibility. I believe it already supports Sphinx > 4: https://github.com/pydata/pydata-sphinx-theme/blob/3ac01cf378b4f58f8c4c411a82fb85324c909ef0/pyproject.toml#L22-L28

maybe @AakashGfude can wrap this into the cruft-removing PR that he is taking over

• https://github.com/executablebooks/sphinx-book-theme/pull/640

[mmcky]

• [x] waiting on new release of sphinx-book-theme before finalising

[choldgraf]

OK we now have the latest version of the book theme in pre-release:

• https://github.com/executablebooks/sphinx-book-theme/pull/640

I've updated setup.py to install the pre-release from the docs, and went ahead and bumped to Sphinx 6 support as well. I'm starting to feel more that we should just drop the upper bound of Sphinx, it is really tedious to have to keep bumping versions across all of these repositories. But that's a conversation for another day.

I think we should merge this and make a release if the tests pass and the docs look good. They look normal to me so I'm gonna merge.

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