yet another attempt to fix sphinx docs build: use sphinx >=2.4

docs/conf.py

@@ -22,6 +22,9 @@
 
 # -- General configuration ------------------------------------------------
 
+# [We specify Sphinx version with the docs extra in requirements.txt, and tell
+# RTD to use it in .readthedocs.yaml. We *don't* use this, which doesn't
+# actually tell RTD to use this version, it just fails if it doesn't match.]
 # If your documentation needs a minimal Sphinx version, state it here.
 #needs_sphinx = '1.0'
 

docs/requirements.txt

@@ -0,0 +1 @@
+sphinx~=2.4

readthedocs.yml

@@ -1,11 +1,8 @@
 # Config file for Sphinx build on ReadTheDocs:
-# https://docs.readthedocs.io/en/latest/yaml-config.html
-#
-# extra_requirements is the important one. docs/conf.py configures sphinx to use
-# autodoc, which generates docs from docstrings in code. It imports the code,
-# which depends on the App Engine SDK.
+# https://docs.readthedocs.io/en/stable/config-file/
+# https://docs.readthedocs.io/en/stable/config-file/v2.html
+version: 2
 python:
-    setup_py_install: false
-    pip_install: true
-    extra_requirements:
-        - appengine-sdk
+   install:
+      - requirements: requirements.txt
+      - requirements: docs/requirements.txt