Gate f-string struct size test for Rustc < 1.76 (#10371)

Closes #10319.

crates/ruff_python_ast/src/nodes.rs

@@ -4148,7 +4148,8 @@ mod tests {
         assert_eq!(std::mem::size_of::<ExprDict>(), 56);
         assert_eq!(std::mem::size_of::<ExprDictComp>(), 48);
         assert_eq!(std::mem::size_of::<ExprEllipsisLiteral>(), 8);
-        assert_eq!(std::mem::size_of::<ExprFString>(), 48);
+        // 56 for Rustc < 1.76
+        assert!(matches!(std::mem::size_of::<ExprFString>(), 48 | 56));
         assert_eq!(std::mem::size_of::<ExprGenerator>(), 48);
         assert_eq!(std::mem::size_of::<ExprIf>(), 32);
         assert_eq!(std::mem::size_of::<ExprIpyEscapeCommand>(), 32);