Changed dependencies with Sphinx 5

[mgeier]

See sphinx-doc/sphinx#10692

As a work-around, Sphinx 4.5.0 (with docutils 0.17.1) can be used.

Once this is solved, #44 should be reverted.

[mgeier]

This will be fixed in Sphinx 5.2.0, see sphinx-doc/sphinx#10855.

#48 will relax the version restriction from #44.