Clean groupby error message (#28324)

Author: dsaxton

pandas/core/groupby/ops.py

@@ -463,9 +463,7 @@ def _cython_operation(self, kind, values, how, axis, min_count=-1, **kwargs):
         # categoricals are only 1d, so we
         # are not setup for dim transforming
         if is_categorical_dtype(values) or is_sparse(values):
-            raise NotImplementedError(
-                "{} are not support in cython ops".format(values.dtype)
-            )
+            raise NotImplementedError("{} dtype not supported".format(values.dtype))
         elif is_datetime64_any_dtype(values):
             if how in ["add", "prod", "cumsum", "cumprod"]:
                 raise NotImplementedError(