Fix ValueError message formatting in int2str
Hi!

I bumped into this particular `ValueError` during my work (because an instance of `np.int64` was passed instead of regular Python `int`), and so I had to `print(type(values))` myself. Apparently, it's just the missing `f` to make message an f-string.

It ain't much for a contribution, but it's honest work. Hope it spares someone else a few seconds in the future 😃