Self-intersection handling

[sergiev]

Sometimes you may occur labels that shapely considers invalid even if they are generally useful.
Here's my example: two closest points were placed in such order that in terms of shapely it's self-intersection and therefore attempt to get_intersection leads to shapely.errors.GEOSException

I propose a little workaround for such case: check validity and call make_valid if needed.

[fcakyon]

@sergiev thanks a lot for your contribution! We can merge this once the tests pass 👍