modernize readthedocs config to get it to build again

.readthedocs.yaml

@@ -5,10 +5,17 @@
 # Required
 version: 2
 
+build:
+  os: "ubuntu-22.04"
+  tools:
+    python: "3.7"
+
 # Build documentation in the docs/ directory with Sphinx
 sphinx:
   configuration: docs/source/conf.py
 
-# Optionally set the version of Python and requirements required to build your docs
+# We recommend specifying your dependencies to enable reproducible builds:
+# https://docs.readthedocs.io/en/stable/guides/reproducible-builds.html
 python:
-  version: 3.7
+  install:
+  - requirements: docs/requirements.txt

docs/requirements.txt

@@ -0,0 +1,4 @@
+# Defining the exact version will make sure things don't break
+sphinx==5.3.0
+sphinx_rtd_theme==1.1.1
+readthedocs-sphinx-search==0.3.2