Inspect arg as Python object, instead of using PyErr_Clear()

[radarhere]

In _convert_transparent(), instead of failing a call to PyArg_ParseTuple(), calling PyErr_Clear() afterwards and then trying again,

Pillow/src/_imaging.c

Lines 1084 to 1089 in 095bbc3

	if (PyArg_ParseTuple(args, "s(iii)", &mode_name, &r, &g, &b)) {
	const ModeID mode = findModeID(mode_name);
	return PyImagingNew(ImagingConvertTransparent(self->image, mode, r, g, b));
	}
	PyErr_Clear();
	if (PyArg_ParseTuple(args, "si", &mode_name, &r)) {

an alternative strategy would be to parse the second argument as a PyObject ("O") and then check if it is a tuple or not.

A similar change could be made in _convert_matrix().