NFL box score over under breaks from flatten links

[phillynch7]

https://github.com/mdgoldberg/sportsref/blob/09f11ac856a23c96d666d1d510bb35d6f050b5c3/sportsref/utils.py#L249

I was merging/testing some of your changes with my fork and it seems the line above breaks the BoxScore.over_under(). The strip removes the space that the over_under splits on later.

I'm not familiar with what the .strip() here solves, so just thought I'd give a heads up on the piece that it breaks.