Fix f string expr split after

[MeGaGiGaGon]

Description

Fixes #4677

My assumption on how the issue started is from a documentation issue on iter_fexpr_spans - the docs said "Spans are half-open ranges (left inclusive, right exclusive)", but it actually returns closed ranges. I assume the author of _get_illegal_split_indices then saw that, and added one to the range, making the ranges include the char after the f-string expr.

To fix this, I just removed that +1 on the ranges.

I looked at the other usages of iter_fexpr_spans, but all of them seem to properly use the ranges as closed.

I updated the tests that were changed by this, it was a small subsection that looks fine, and since no other code tests were effected that supports that everything else uses the spans as closed. I also fixed the docs for iter_fexpr_spans to prevent this in the future.

Checklist - did you ...

• Add an entry in CHANGES.md if necessary?
• Add / update tests if necessary?
• Add new / update outdated documentation?