Apply ruff/flake8-comprehensions rule C401

C401 Unnecessary generator (rewrite as a `set` comprehension)

Author: DimitriPapadopoulos

numcodecs/tests/test_registry.py

@@ -26,7 +26,7 @@ def test_all_classes_registered():
 
     see #346 for more info
     """
-    missing = set(
+    missing = {
         obj.codec_id
         for _, submod in inspect.getmembers(numcodecs, inspect.ismodule)
         for _, obj in inspect.getmembers(submod)
@@ -36,7 +36,7 @@ def test_all_classes_registered():
             and obj.codec_id not in numcodecs.registry.codec_registry
             and obj.codec_id is not None  # remove `None`
         )
-    )
+    }
 
     if missing:
         raise Exception(f"these codecs are missing: {missing}")  # pragma: no cover