Changing how we install poetry dependencies on ReadTheDocs. Changing …

…to follow readthedocs/readthedocs.org#4912 (comment) which seems to be the recommended way. Previous method was using mkdocs version 1.1 for some reason. mkdocs version 1.1 doesn't work with the latest version of jinja2 (https://twitter.com/readthedocs/status/1507388916013314048).

.readthedocs.yaml

@@ -6,13 +6,10 @@ build:
   os: ubuntu-20.04
   tools: {python: "3.9"}
   jobs:
-    pre_create_environment:
-      - asdf plugin add poetry
-      - asdf install poetry latest
-      - asdf global poetry latest
-      - poetry config virtualenvs.create false
     post_install:
-      - poetry install
+      - pip install poetry==1.2.0
+      - poetry config virtualenvs.create false
+      - poetry install --with doc
 
 # Build documentation in the docs/ directory with Sphinx
 mkdocs: