It is not a pandas problem, historical data for CU13.CBT is not available, you can check that here you wont find the link to historical prices (compare with this).
Try with another symbol and it should work. Example:
>>> import pandas.io.data as web
>>> start = datetime.datetime(2013, 8, 8)
>>> end = datetime.datetime(2013, 8, 10)
>>> f = web.DataReader("F", 'yahoo', start, end)
>>> f
Open High Low Close Volume Adj Close
Date
2013-08-08 16.94 17.03 16.87 16.98 26589500 16.98
2013-08-09 16.95 17.11 16.94 17.02 25625300 17.02