Allow whitespace around colon in slices for `whitespace-before-punctuation` (`E203`) (#8654)
## Summary
This PR makes `whitespace-before-punctuation` (`E203`) compatible with
the formatter by relaxing the rule a bit, as compared to the pycodestyle
implementation. It's also more consistent with PEP 8, which says:
> However, in a slice the colon acts like a binary operator, and should
have equal amounts on either side (treating it as the operator with the
lowest priority).
Closes https://github.com/astral-sh/ruff/issues/7259.
Closes https://github.com/astral-sh/ruff/issues/8642.
## Test Plan
`cargo test`