Fix optional sphinx dependency (#44)

.github/workflows/test.yml

@@ -33,7 +33,7 @@ jobs:
     - name: Install dependencies
       run: |
         python -m pip install --upgrade pip
-        pip install -e ".[dev]"
+        pip install -e . --group dev
 
     # - name: Run mypy
     #   run: |
@@ -52,4 +52,4 @@ jobs:
     #   uses: actions/upload-artifact@v3
     #   with:
     #     name: dist-${{ matrix.python-version }}
-    #     path: dist/
+    #     path: dist/