Build the documentation in CI.

Even though we aren't uploading it, this *should* make sure that the
readthedocs build works.

.github/workflows/testing.yml

@@ -23,6 +23,7 @@ jobs:
       - name: Flake8
         run: |
           flake8 sprockets tests.py
+
   test:
     runs-on: ubuntu-latest
     strategy:
@@ -53,3 +54,19 @@ jobs:
           file: ./coverage.xml
           flags: unittests
           fail_ci_if_error: true
+
+  docs:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+      - name: Set up Read the Docs compatible python
+        uses: actions/setup-python@v2
+        with:
+          python-version: 3.7
+      - name: Install dependencies
+        run: |
+          python -m pip install '.[docs]'
+          python -m pip install .
+      - name: Build documentation
+        run: |
+          ./setup.py build_sphinx