Fix numpy deprecation by replacing np.unicode_ with np.str_ in type check

Author: lwgray

yellowbrick/contrib/missing/dispersion.py

@@ -97,7 +97,7 @@ def get_nan_locs(self, **kwargs):
         the coordinates in the matrix
         """
         if np.issubdtype(self.X.dtype, np.bytes_) or np.issubdtype(
-            self.X.dtype, np.unicode_
+            self.X.dtype, np.str_
         ):
             mask = np.where(self.X == "")
             nan_matrix = np.zeros(self.X.shape)