Ensure documentation builds use Sphinx 5.3

There are some issues with Sphinx 6.x, docutils, and sphinx-rtd-theme.

requirements-rtd.txt

@@ -2,5 +2,5 @@
 # See https://github.com/sphinx-doc/sphinx/issues/9727
 # and https://github.com/readthedocs/sphinx_rtd_theme/issues/1323
 docutils<0.18
-sphinx>=2.0
-sphinx-rtd-theme>=1.0
+sphinx~=5.3
+sphinx-rtd-theme~=1.0