CLN: Replace confusing brackets with backticks
There are some errors which read like:

> only list-like objects are allowed to be passed to isin(), you
> passed a [str]

(note how the type `str` is enclosed in square brackets)

This is potentially confusing, since some languages use notation like `[TYPE]` to denote a list of TYPE objects. So the user could read the error message saying they can't use `[str]`, but intepret `[str]` to mean "a list of str objects".

This commit removes the possibility of confusion by using backticks instead of square brackets.
Thanks @beyarkay  Thanks for the review!