Fix Read the Docs build environment (#887)

* Fix ReadTheDocs build environment

Pin the poetry version to 1.4.2, which is known to work well with ReadTheDocs
Explicitly install virtualenv 20.23.1, which doesn't have the issue with the 'free_threaded' attribute

* Bump Poetry to 1.5.1

Poetry 1.5.1 should support dependency groups (spamming the autograder)

.readthedocs.yml

@@ -8,7 +8,8 @@ build:
     python: '3.10'
   jobs:
     post_create_environment:
-      - pip install poetry myst-parser
+      - pip install poetry==1.5.1 myst-parser
+      - pip install virtualenv==20.23.1
     post_install:
       # VIRTUAL_ENV needs to be set manually for now.
       # See https://github.com/readthedocs/readthedocs.org/pull/11152/