ci: re-enable mypy in altair (#3503)

Author: MarcoGorelli

.github/workflows/downstream_tests.yml

@@ -50,12 +50,10 @@ jobs:
         run: |
           cd altair
           pytest tests
-      # Temporarily commented-out to get CI green
-      # https://github.com/narwhals-dev/narwhals/issues/3013
-      # - name: Run mypy
-      #   run: |
-      #     cd altair
-      #     mypy altair tests
+      - name: Run mypy
+        run: |
+          cd altair
+          mypy altair tests
 
   marimo:
     strategy: