.github/workflows/downstream.yml: Test with sagemath-repl (#14317)

Adding a lightweight test with SageMath using the modularized
distribution https://pypi.org/project/sagemath-repl/ of the Sage
library.

Motivated by sagemath/sage#36975,
#14223 (comment),
sagemath/sage#36993,
sagemath/sage#37031 @Carreau

.github/workflows/downstream.yml

@@ -44,9 +44,26 @@ jobs:
         # we must install IPython after ipykernel to get the right versions.
         python -m pip install --upgrade --upgrade-strategy eager flaky ipyparallel
         python -m pip install --upgrade 'pytest<7'  'pytest_asyncio<0.21'
-    - name: pytest
+    - name: pytest ipykernel
       env:
         COLUMNS: 120
       run: |
         cd ../ipykernel
         pytest
+    - name: Install sagemath-repl
+      run: |
+        cd ..
+        git clone --depth 1 https://github.com/sagemath/sage
+        cd sage
+        # We cloned it for the tests, but for simplicity we install the
+        # wheels from PyPI.
+        # (Avoid 10.3b6 because of https://github.com/sagemath/sage/pull/37178)
+        pip install --pre "sagemath-repl<10.3b6" "sagemath-environment<10.3b6"
+        # Install optionals that make more tests pass
+        pip install sagemath-categories pillow
+        cd ..
+    - name: Test sagemath-repl
+      run: |
+        cd ../sage/
+        # From https://github.com/sagemath/sage/blob/develop/pkgs/sagemath-repl/tox.ini
+        sage-runtests -p --environment=sage.all__sagemath_repl --baseline-stats-path=pkgs/sagemath-repl/known-test-failures.json --initial --optional=sage src/sage/repl src/sage/doctest src/sage/misc/sage_input.py src/sage/misc/sage_eval.py