try without '

.readthedocs.yaml

@@ -8,7 +8,7 @@ build:
     post_create_environment:
       # Install poetry
       # https://python-poetry.org/docs/#installing-manually
-      - pip install 'poetry<3.0.0'
+      - pip install poetry<3.0.0
     post_install:
       # Install dependencies with 'docs' dependency group
       # https://python-poetry.org/docs/managing-dependencies/#dependency-groups