Remove exception-handler lexing from unused-bound-exception fix (#5851)

## Summary

The motivation here is that it will make this rule easier to rewrite as
a deferred check. Right now, we can't run this rule in the deferred
phase, because it depends on the `except_handler` to power its autofix.
Instead of lexing the `except_handler`, we can use the `SimpleTokenizer`
from the formatter, and just lex forwards and backwards.

For context, this rule detects the unused `e` in:

```python
try:
  pass
except ValueError as e:
  pass
```

crates/ruff_python_formatter/src/statement/stmt_with.rs

@@ -3,13 +3,13 @@ use rustpython_parser::ast::{Ranged, StmtAsyncWith, StmtWith, Suite, WithItem};
 
 use ruff_formatter::{format_args, write, FormatError};
 use ruff_python_ast::node::AnyNodeRef;
+use ruff_python_whitespace::{SimpleTokenizer, TokenKind};
 
 use crate::comments::trailing_comments;
 use crate::expression::parentheses::{
     in_parentheses_only_soft_line_break_or_space, optional_parentheses,
 };
 use crate::prelude::*;
-use crate::trivia::{SimpleTokenizer, TokenKind};
 use crate::FormatNodeRule;
 
 pub(super) enum AnyStatementWith<'a> {