Fix Python verion in Wagtail CI step

This was the only step that is using 3.8, while the Docker image and all
other steps are using 3.9.9.

This caused the CI to fail when using type annotations like `list[int]`,
because using the literal types is only allowed in 3.9+.

.github/workflows/continous-integration.yml

@@ -70,7 +70,7 @@ jobs:
       - uses: actions/checkout@v2
       - uses: actions/setup-python@v2
         with:
-          python-version: 3.8
+          python-version: 3.9.9
           cache: "pip"
       - uses: actions/setup-node@v2
         with: