Trac #21203 review issue 10: use "raise ... from None" where approriate

Author: dkrenn

src/sage/combinat/k_regular_sequence.py

@@ -362,4 +362,4 @@ def _n_to_index_(self, n):
         try:
             return W(n.digits(self.k))
         except OverflowError:
-            raise ValueError('value {} of index is negative'.format(n))
+            raise ValueError('value {} of index is negative'.format(n)) from None