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.

Update: Wrong, the spans are actually half-open, and I also got confused by it like the original author.

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 as half-open.

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 as half-open. 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• Add new / update outdated documentation?

[hauntsaninja]

I think the fix is right, but the change to the comment is wrong

[hauntsaninja]

this isn't right, the spans are half-open ranges. See e.g.

black/tests/test_trans.py

Line 26 in 8310a11

check("""text {var} text""", [(5, 10)], ["{var}"])

[hauntsaninja]

>>> """text {var} text"""[5]
'{'
>>> """text {var} text"""[10]
' '
>>> """text {var} text"""[5:10]
'{var}'

[MeGaGiGaGon]

Looks like I also got confused by the ranges 😓 fixed