scripts: Fixed openio issue where some scripts didn't import os

This only failed if "-" was used as an argument (for stdin/stdout), so
the issue was pretty hard to spot.

openio is a heavily copy-pasted function, so it makes sense to just add
the import os to openio directly. Otherwise this mistake will likely
happen again in the future.

scripts/plot.py

@@ -134,6 +134,7 @@ def si2(x, w=5):
 
 def openio(path, mode='r', buffering=-1):
     # allow '-' for stdin/stdout
+    import os
     if path == '-':
         if 'r' in mode:
             return os.fdopen(os.dup(sys.stdin.fileno()), mode, buffering)