ReadTheDocs: update and fix (#906)

commit-id:f9f512d4

.readthedocs.yml

@@ -5,6 +5,11 @@
 # Required
 version: 2
 
+build:
+  os: ubuntu-22.04
+  tools:
+    python: "3.9"
+
 # Build documentation in the docs/ directory with Sphinx
 sphinx:
   builder: html
@@ -17,13 +22,8 @@ formats:
 
 # Optionally set the version of Python and requirements required to build your docs
 python:
-  version: 3.7
   install:
-    - requirements: requirements.txt
-    - requirements: docs/requirements.txt
     - method: pip
       path: .
-# Conda lets us get up-to-date binaries that are not available in Ubuntu installs on RTD.
-# e.g., Pandoc.
-conda:
-  environment: docs/environment.yml
+      extra_requirements:
+      - dev