I'm trying to calculate the pct\_change in a Pandas series but not in the regular way in which changes are calculated between adjacent elements, but within elements two places apart.

For instance: 

```
pd.Series([1, 2, 3, 4, 5, 6, 7])

0    1
1    2
2    3
3    4
4    5
5    6
6    7
dtype: int64

```

Compare `1` with `4`, `2` with `5`, `3` with `6` and `4` with `7` (strides of 3).