Avoid dict normalization in __dask_tokenize__ (#15187)

There are  currently [CI failures](https://github.com/rapidsai/cudf/actions/runs/8089269486/job/22105880070?pr=15181#step:7:1050) that seem to be caused by non-deterministic `dict` normalization in `Frame.__dask_tokenize__`. This PR avoids normalizing that dictionary.

Authors:
  - Richard (Rick) Zamora (https://github.com/rjzamora)

Approvers:
  - Bradley Dice (https://github.com/bdice)

URL: #15187

python/cudf/cudf/core/frame.py

@@ -1958,7 +1958,7 @@ def __dask_tokenize__(self):
 
         return [
             type(self),
-            normalize_token(self._dtypes),
+            str(self._dtypes),
             normalize_token(self.to_pandas()),
         ]