Let .sel take tolerance for datetime or str time

[MarcoGorelli]

follow up to #339

[khider]

@MarcoGorelli The examples use days for the tolerance. Is it possible to do it with years (I know we had to create something like this for resample) or we're stuck with the "highest" pandas units?

[MarcoGorelli]

It's that the standard library timedelta only has years as the highest unit https://docs.python.org/3/library/datetime.html#datetime.timedelta

I think it's because 'years' wouldn't be a fixed amount of time