Fix Sphinx requirement and update it

Author: willkg

.readthedocs.yaml

@@ -10,7 +10,9 @@ version: 2
 build:
   os: ubuntu-20.04
   tools:
-    python: "3.8"
+    # Build with 3.9 because this is the minimum version Sphinx 7 can work
+    # with.
+    python: "3.9"
 
 # Build documentation in the docs/ directory with Sphinx
 sphinx:

requirements-dev.txt

@@ -10,5 +10,4 @@ tox-gh-actions==3.1.3
 twine==4.0.2
 wheel==0.41.2
 
-# NOTE(willkg): Held back until we drop support for Python 3.7
-Sphinx==5.3.0
+Sphinx==7.2.6; python_version > "3.8"