Studies on Pyridonecarboxylic Acids. Part 5. A Practical Synthesis of Ethyl 6,7-Difluoro-1-methyl-4-oxo-4H-[1,3]thiazeto [3,2-a]quinoline-3-carboxylate, a Key Intermediate for the New Tricyclic Quinolone, Prulifloxacin (NM441) and Ver