Updated ImagePath tolist() default

[radarhere]

ImagePath.Path.tolist() is a C function that we have exposed in our public Python API. It has an argument flat that it uses like so -

Pillow/src/path.c

Lines 431 to 440 in b073a89

	path_tolist(PyPathObject *self, PyObject *args) {
	PyObject *list;
	Py_ssize_t i;
	int flat = 0;
	if (!PyArg_ParseTuple(args, "|i:tolist", &flat)) {
	return NULL;
	}
	if (flat) {

Looking at the documentation for ImagePath.Path.tolist(), https://pillow.readthedocs.io/en/stable/reference/ImagePath.html#PIL.ImagePath.PIL.ImagePath.Path.tolist, flat is documented as

flat – By default, this function returns a list of 2-tuples [(x, y), …]. If this argument is True, it returns a flat list [x, y, …] instead.

Ok, so flat is a boolean. That's how it should be thought of, and how it is used.

Except the documentation at that link also the default for flat as 0, which could cause a brief moment of confusion as to why the default is a different type.

The reason isn't not False would be because the code is in C, not Python, but I think it would be clearer if we described our Python API as having a default of False.

[hugovk]

I added the documentation label, but then saw you added and removed it too! Feel free to remove it again if you like :)

[radarhere]

It was initially documentation only, but then I technically changed a test too in the second commit, so... whatever works.