Make codespell a docs dependency

.github/workflows/spellcheck.yml

@@ -20,6 +20,6 @@ jobs:
           ${{ runner.os }}-pip-
     - name: Install dependencies
       run: |
-        pip install codespell
+        pip install -e '.[docs]'
     - name: Check spelling
       run: codespell docs/*.rst --ignore-words docs/codespell-ignore-words.txt

setup.py

@@ -65,7 +65,7 @@ setup(
     """,
     setup_requires=["pytest-runner"],
     extras_require={
-        "docs": ["sphinx_rtd_theme", "sphinx-autobuild"],
+        "docs": ["sphinx_rtd_theme", "sphinx-autobuild", "codespell"],
         "test": [
             "pytest>=5.2.2,<6.3.0",
             "pytest-xdist>=2.2.1,<2.4",