With the GIL, for an int i, `i += 1` is not thread-safe, but IIRC for a list l, `l += [1]` (i.e. extend) is. Presumably this patch changes the list implementat

With the GIL, for an int i, `i += 1` is not thread-safe, but IIRC for a list l, `l += [1]` (i.e. extend) is. Presumably this patch changes the list implementation in some way so that the extend operation remains thread-safe without the GIL.