Infra: Skip Sphinx 7.3 to fix missing 404s (#3758)

Author: hugovk

requirements.txt

@@ -2,7 +2,7 @@
 Pygments >= 2.9.0
 # Sphinx 6.1.0 broke copying images in parallel builds; fixed in 6.1.2
 # See https://github.com/sphinx-doc/sphinx/pull/11100
-Sphinx >= 5.1.1, != 6.1.0, != 6.1.1
+Sphinx >= 5.1.1, != 6.1.0, != 6.1.1, < 7.3
 docutils >= 0.19.0
 
 sphinx-autobuild