Assuming that the highlighting has happened via an annotation:

• loop over the annotations of the page (downselecting to appropriate annot type)
• take the annotation rectangle annot.rect and enlarge a little. This is recommended, because highlighting by many software packages create a too small annot rectangle to completely cover the complete text. So add an allowance like rect = annot.rect + (-5, -5, 5, 5) - which is 5 pixels large in every direction.
• Extract text like so text = page.get_text(clip=rect).

If highlighting has not happened via annotations, things are much more complicated, so let's hope you will get away with this recipe.