Make the return in disassemble_all explicit

Rather than just a `return` statement, `StopIteration` makes it clearer that the generator is done. They're functionally equivalent, but it's better for clarity in my opinion.

Author: Josh Watson

pyevmasm/evmasm.py

@@ -681,7 +681,7 @@ def disassemble_all(bytecode, pc=0):
     while True:
         instr = disassemble_one(bytecode, pc=pc)
         if not instr:
-            return
+            raise StopIteration
         pc += instr.size
         yield instr