docs: replace docs extra by Poetry group (#330)

Committed via https://github.com/asottile/all-repos

.readthedocs.yml

@@ -9,15 +9,16 @@ build:
   os: ubuntu-20.04
   tools:
     python: "3.9"
+  jobs:
+    post_create_environment:
+      # Install poetry
+      - pip install poetry
+      # Tell poetry to not use a virtual environment
+      - poetry config virtualenvs.create false
+    post_install:
+      # Install dependencies
+      - poetry install --with docs
 
-# Build documentation in the docs/ directory with Sphinx
+# Build documentation in the docs directory with Sphinx
 sphinx:
   configuration: docs/conf.py
-
-# Optionally declare the Python requirements required to build your docs
-python:
-  install:
-    - method: pip
-      path: .
-      extra_requirements:
-        - docs