PyRNA: all optional args now must be keyword args

In some cases the RNA API should be updated to make arguments use the
'required' flag, instead of adjusting Python scripts.

See T47811

release/scripts/startup/bl_ui/properties_data_lightprobe.py

@@ -78,7 +78,6 @@ class DATA_PT_lightprobe(DataButtonsPanel, Panel):
         elif probe.type == 'PLANAR':
             col = layout.column()
             col.prop(probe, "influence_distance", text="Distance")
-            col.prop(probe, "falloff")
         else:
             col = layout.column()
             col.prop(probe, "influence_type")

source/blender/python/intern/bpy_rna.c

@@ -5654,6 +5654,17 @@ static PyObject *pyrna_func_call(BPy_FunctionRNA *self, PyObject *args, PyObject
 		item = NULL;
 
 		if (i < pyargs_len) {
+			/* New in 2.8x, optional arguments must be keywords.  */
+			if (UNLIKELY((flag_parameter & PARM_REQUIRED) == 0)) {
+				PyErr_Format(PyExc_TypeError,
+				             "%.200s.%.200s(): required parameter \"%.200s\" to be a keyword argument!",
+				             RNA_struct_identifier(self_ptr->type),
+				             RNA_function_identifier(self_func),
+				             RNA_property_identifier(parm));
+				err = -1;
+				break;
+			}
+
 			item = PyTuple_GET_ITEM(args, i);
 			kw_arg = false;
 		}