It was added in 8bd6c70d47e16fd81c8e3aefd4b2ec6dd90f38d6 which introduced `_iotools` and `_is_string_like`. 
However, as far as I can see, its usage was removed on the same commit. I couldn't spot any current usage of `_string_like`
within `numpy`, and as it starts with a dash I would assume no outside user uses it. Indeed a comment by @pierregm 
could shed some light.