Cleanup: rename checks for Python struct strings

Using 'format' prefix made this read as if t was for string formatting.

Use 'PyC_StructFmt' prefix instead since these values are compatible
with formatting from Python's 'struct' module.

source/blender/python/gpu/gpu_py_element.c

@@ -92,7 +92,7 @@ static PyObject *bpygpu_IndexBuf_new(PyTypeObject *UNUSED(type), PyObject *args,
 		}
 
 		if (pybuffer.itemsize != 4 ||
-		    PyC_Formatchar_is_floating_type(PyC_Formatchar_get(pybuffer.format)))
+		    PyC_StructFmt_type_is_float_any(PyC_StructFmt_type_from_str(pybuffer.format)))
 		{
 			PyErr_Format(PyExc_ValueError,
 			             "Each index must be an 4-bytes integer value");