Fix documentation build (#1462)

* Don't use extras for the doc build

* Restrict Sphinx version to avoid breakage from Sphinx 4

* Install traits properly

* Restrict Sphinx version

.github/workflows/test-documentation-build.yml

@@ -21,7 +21,8 @@ jobs:
     - name: Install dependencies and local packages
       run: |
         python -m pip install --upgrade pip
-        python -m pip install .[docs]
+        python -m pip install "Sphinx<4" enthought_sphinx_theme
+        python -m pip install .
     - name: Build HTML documentation with Sphinx
       run: |
         cd docs