Replace nonzero occurence in TL

Author: Guillaume Lemaitre

imblearn/under_sampling/tomek_links.py

@@ -155,7 +155,7 @@ def _sample(self, X, y):
         if self.return_indices:
             # Return the indices of interest
             return (X[np.logical_not(links)], y[np.logical_not(links)],
-                    np.nonzero(np.logical_not(links))[0])
+                    np.flatnonzero(np.logical_not(links)))
         else:
             # Return data set without majority Tomek links.
             return X[np.logical_not(links)], y[np.logical_not(links)]