Format StmtAugAssign (#5655)

## Summary

Format statements such as `tree_depth += 1`. This is a statement that
does not allow any line breaks, the only thing to be mindful of is to
parenthesize the assigned expression

Jaccard index on django: 0.915 -> 0.918

## Test Plan

black tests, and two new tests, a basic one and one that ensures that
the child gets parentheses. I ran the django stability check.

crates/ruff_python_formatter/src/statement/stmt_aug_assign.rs

@@ -1,4 +1,6 @@
-use crate::{not_yet_implemented, FormatNodeRule, PyFormatter};
+use crate::expression::parentheses::Parenthesize;
+use crate::{AsFormat, FormatNodeRule, PyFormatter};
+use ruff_formatter::prelude::{space, text};
 use ruff_formatter::{write, Buffer, FormatResult};
 use rustpython_parser::ast::StmtAugAssign;
 
@@ -7,6 +9,22 @@ pub struct FormatStmtAugAssign;
 
 impl FormatNodeRule<StmtAugAssign> for FormatStmtAugAssign {
     fn fmt_fields(&self, item: &StmtAugAssign, f: &mut PyFormatter) -> FormatResult<()> {
-        write!(f, [not_yet_implemented(item)])
+        let StmtAugAssign {
+            target,
+            op,
+            value,
+            range: _,
+        } = item;
+        write!(
+            f,
+            [
+                target.format(),
+                space(),
+                op.format(),
+                text("="),
+                space(),
+                value.format().with_options(Parenthesize::IfBreaks)
+            ]
+        )
     }
 }