ci: pin requirements on Read the Docs (#429)

Author: redeboer

.readthedocs.yml

@@ -8,10 +8,12 @@ sphinx:
 formats:
   - htmlzip
 
-python:
-  version: "3.8"
-  install:
-    - method: pip
-      path: .
-      extra_requirements:
-        - doc
+build:
+  os: ubuntu-20.04
+  tools:
+    python: "3.8"
+  apt_packages:
+    - graphviz
+  jobs:
+    post_install:
+      - pip install -e .[doc] -c .constraints/py3.8.txt