Fix .readthedocs.yml

Following readthedocs/readthedocs.org#11150

Author: ewjoachim

.readthedocs.yml

@@ -14,7 +14,7 @@ build:
   tools:
     python: '3'
   jobs:
+    post_create_environment:
+      - python -m pip install poetry
     post_install:
-      - pip install -U poetry
-      - poetry config virtualenvs.create false
-      - poetry install
+      - VIRTUAL_ENV=$READTHEDOCS_VIRTUALENV_PATH python -m poetry install