CUDF changed API to assume the dtype is already converted to a cudf type (#4955)

Switch from np.dtype to cudf.dtype to accommodate a cudf change.

Author: ChuckHastings

python/cugraph/cugraph/structure/hypergraph.py

@@ -40,7 +40,7 @@
 from cugraph.structure.symmetrize import symmetrize
 
 
-_str_dtype = np.dtype("str")
+_str_dtype = cudf.dtype("str")
 
 
 def hypergraph(