Crashes while selecting "This week"

[JCLemme]

Sometimes, if you let Chartbot select the most recent week for you, it crashes. This seems to be an issue with the way billboard.py handles dates. Billboard charts are assigned by the week, like "week of 03/02/2019" and such, and when you feed billboard.py a date between charts it'll snap to the closest published chart to that date. It looks like that snapping is broken for the most recent chart available.