scripts: Added missing sys imports

Unfortunately the import sys in the argparse block was hiding missing
sys imports.

The mistake was assuming the import sys in Python would limit the scope
to that if block, but Python's late binding strikes again...

scripts/plotmpl.py

@@ -24,6 +24,7 @@ import numpy as np
 import os
 import shlex
 import shutil
+import sys
 import time
 
 import matplotlib as mpl