python stub for pack has incorrect type

[dshekter]

The stub generator for the Pack function

flatbuffers/src/idl_gen_python.cpp

Line 382 in bab1075

stub << " def Pack(self, builder: flatbuffers.Builder) -> None: ...\n";

says Pack returns None, but it returns an integer since it returns the value from end End function which is an int (aka uoffset).