chore: tidy changelog fragments and test docstring for the VerboseModule removal

changes/4505.bugfix.md

@@ -0,0 +1 @@
+The `zarr.storage` module can now be serialized with `cloudpickle`. Previously, code that referenced the module (for example a Dask task that used `zarr.storage.ObjectStore`) failed with `TypeError: cannot pickle 'VerboseModule' object`.

changes/4505.removal.md

@@ -0,0 +1 @@
+Setting `zarr.storage.default_compressor` no longer emits a deprecation warning. The attribute has had no effect since 3.0.0; configure `array.v2_default_compressor` through `zarr.config` instead.

tests/test_store/test_core.py

@@ -439,12 +439,11 @@ def test_different_open_mode(tmp_path: Path) -> None:
 
 
 def test_storage_module_is_picklable() -> None:
-    # `zarr.storage` used to be swapped for a `VerboseModule` subclass of
-    # `types.ModuleType` in order to warn about the long-deprecated
-    # `zarr.storage.default_compressor` attribute. cloudpickle dispatches on the
-    # exact type of an object, so the subclass was not registered and
-    # serializing the module raised `TypeError: cannot pickle 'VerboseModule'
-    # object`, which broke passing `zarr.storage.ObjectStore` around with Dask.
-    # See issue #4029.
+    """
+    `zarr.storage` is a plain `types.ModuleType` and round-trips through `cloudpickle`.
+
+    Serializers such as `cloudpickle` dispatch on the exact type of an object, so a
+    `ModuleType` subclass would not be serializable. See issue #4029.
+    """
     assert type(zarr.storage) is ModuleType
     assert cloudpickle.loads(cloudpickle.dumps(zarr.storage)) is zarr.storage