Add simple test for objects namespace

tests/test_objects.py

@@ -0,0 +1,14 @@
+import seaborn.objects
+from seaborn._core.plot import Plot
+from seaborn._core.moves import Move
+from seaborn._core.scales import Scale
+from seaborn._marks.base import Mark
+from seaborn._stats.base import Stat
+
+
+def test_objects_namespace():
+
+    for name in dir(seaborn.objects):
+        if not name.startswith("__"):
+            obj = getattr(seaborn.objects, name)
+            assert issubclass(obj, (Plot, Mark, Stat, Move, Scale))