Add support for newer docutils and sphinx>=6.0.0

[hagenw]

At the moment we require docutils<0.17 to make sure bullet points don't look like this:

Starting with sphinx>=6.0.0 they have docutils>=0.18, which means we cannot use that version of sphinx.

A few remarks from docutils discussions at sphinx:

• At Update Sphinx to work with docutils-0.18.x sphinx-doc/sphinx#9777 (comment) they describe which CSS entries need to be updated for newer docutils as with sphinx==5.0.0

[hagenw]

With Python 3.13 we will need at least sphinx 6.2, as the imghdr module was removed from the standard library in Python 3.13 (and not published as a package for pip), but that module is needed for sphinx<6.2.
Compare python/cpython#104818.