Revert "Require sphinx<7.3.0 (#2404)"

This reverts commit 6bbcde2.

Fixed in sphinx 7.3.6 (https://www.sphinx-doc.org/en/master/changes.html)

documentation/rtd_requirements.txt

@@ -1,7 +1,5 @@
 # NOTE: relative paths are expected to be relative to the repository root
-
-# sphinx<7.3.0: https://github.com/AMICI-dev/AMICI/issues/2403
-sphinx<7.3.0
+sphinx
 mock>=5.0.2
 setuptools>=67.7.2
 pysb>=1.11.0