CI: Fix pip install problem with Pygments version

Recently `python setup.py build_sphinx` started failing with:

    pkg_resources.VersionConflict: (Pygments 2.3.1
    (/usr/lib/python3/dist-packages), Requirement.parse('Pygments>=2.12'))

The reason is that `doc8` 1.0.0 installs `Pygments` 2.3.1, then `Sphinx`
5.3.0 needs `Pygments` ≥ 2.12.

The easiest fix is to change the install order.

.github/workflows/ci.yaml

@@ -21,8 +21,8 @@ jobs:
       - name: Set up Python
         uses: actions/setup-python@v4
       - run:
-          python -m pip install flake8 flake8-import-order doc8 sphinx
+          python -m pip install flake8 flake8-import-order sphinx
-            rstcheck[sphinx]
+            rstcheck[sphinx] doc8
       - run: python -m pip install .
       - run: flake8 .
       - run: doc8 $(git ls-files '*.rst')