Revert "Only check docs on Travis"

This reverts commit fbb14f67a3.

.github/workflows/test.yml

@@ -95,6 +95,12 @@ jobs:
         name: errors
         path: Tests/errors
 
+    - name: Docs
+      if: matrix.python-version == 3.8
+      run: |
+        pip install sphinx-rtd-theme
+        make doccheck
+
     - name: After success
       if: success()
       run: |