Fix 128-bit int regression on big-endian with Python <3.13

Fixes PyO3#4290.

src/conversions/std/num.rs

@@ -272,8 +272,10 @@ mod fast_128bit_int_conversion {
                                 "Python int larger than 128 bits",
                             ));
                         }
+                        Ok(<$rust_type>::from_ne_bytes(buffer))
                     }
-                    Ok(<$rust_type>::from_ne_bytes(buffer))
+                    #[cfg(not(Py_3_13))]
+                    Ok(<$rust_type>::from_le_bytes(buffer))
                 }
 
                 #[cfg(feature = "experimental-inspect")]