install codespell properly

Author: simonw

.github/workflows/spellcheck.yml

@@ -15,7 +15,7 @@ jobs:
         cache-dependency-path: pyproject.toml
     - name: Install dependencies
       run: |
-        pip install -e '.[docs]'
+        pip install . --group dev
     - name: Check spelling
       run: |
         codespell docs/*.rst --ignore-words docs/codespell-ignore-words.txt