Try poetry install for readthedocs

- as described here:
https://docs.readthedocs.io/en/latest/build-customization.html#install-dependencies-with-poetry
and
readthedocs/readthedocs.org#11152

Author: davewith

.readthedocs.yaml

@@ -1,13 +1,16 @@
 version: 2
 
 build:
-    os: ubuntu-20.04
+    os: ubuntu-22.04
     tools:
-        python: "3.10"
+        python: "3.11"
+    jobs:
+        post_create_environment:
+            - pip install poetry
+        post_install:
+            # VIRTUAL_ENV needs to be set manually for now.
+            # See https://github.com/readthedocs/readthedocs.org/pull/11152/
+            - VIRTUAL_ENV=$READTHEDOCS_VIRTUALENV_PATH poetry install
 
 sphinx:
     configuration: doc/source/conf.py
-
-python:
-    install:
-        - requirements: requirements.txt