Fix panic with 8 in octal escape (#8356)

**Summary** The digits for an octal escape are 0 to 7, not 0 to 8,
fixing the panic in #8355

**Test plan** Regression test parser fixture

crates/ruff_python_parser/src/snapshots/ruff_python_parser__string__tests__dont_panic_on_8_in_octal_escape.snap

@@ -0,0 +1,28 @@
+---
+source: crates/ruff_python_parser/src/string.rs
+expression: parse_ast
+---
+[
+    Assign(
+        StmtAssign {
+            range: 0..16,
+            targets: [
+                Name(
+                    ExprName {
+                        range: 0..4,
+                        id: "bold",
+                        ctx: Store,
+                    },
+                ),
+            ],
+            value: StringLiteral(
+                ExprStringLiteral {
+                    range: 7..16,
+                    value: "\u{3}8[1m",
+                    unicode: false,
+                    implicit_concatenated: false,
+                },
+            ),
+        },
+    ),
+]