FutureWarning: pandas indexing

```
FutureWarning: The behavior of `series[i:j]` with an integer-dtype index is deprecated. In a future version, this will be treated as *label-based* indexing, consistent with e.g. `series[i]` lookups. To retain the old behavior, use `series.iloc[i:j]`. To get the future behavior, use `series.loc[i:j]`.
  self.Z[row][i0 : i1 + 1] = _exchanges[i0 : i1 + 1]
```

