Pin poetry version as a workaround for poetry export plugin requirement

https://python-poetry.org/docs/cli/#export

.readthedocs.yml

@@ -8,7 +8,7 @@ build:
     pre_build:
       - cp -r examples docs/source/
     post_install:
-      - pip install --no-cache-dir poetry
+      - pip install --no-cache-dir poetry==1.8.5
       - poetry export -f requirements.txt -o requirements.txt -E all --without-hashes
       - pip install --no-cache-dir -r requirements.txt