set virtualenv for readthedocs builds

.readthedocs.yml

@@ -16,8 +16,9 @@ build:
       - poetry config virtualenvs.create false
     post_install:
       # Install dependencies with 'docs' dependency group
-      # https://python-poetry.org/docs/managing-dependencies/#dependency-groups
-      - poetry install --with dev,docs --all-extras
+      # (https://python-poetry.org/docs/managing-dependencies/#dependency-groups)
+      # and leverage READTHEDOCS_VIRTUALENV_PATH for environment.
+      - VIRTUAL_ENV=$READTHEDOCS_VIRTUALENV_PATH poetry install --with dev,docs --all-extras
 
 sphinx:
   builder: html