enh: add sprint unique contributor counts to sprint page

This page
https://www.pyopensci.org/metrics/contributors/sprints.html 

has a nice overview of total sprint contributions but we also need unique contributors for each year and a total sprinters involved at the top.

[the data for this can be found here: ](https://github.com/pyOpenSci/metrics/blob/main/_data/sprint_data.csv)