Update classtree.py

linux:~>> classtree.py
[(<type 'object'>, ()),
 [(<type 'exceptions.BaseException'>, (<type 'object'>,)),
  [(<type 'exceptions.Exception'>, (<type 'exceptions.BaseException'>,)),
   [(<type 'exceptions.StandardError'>, (<type 'exceptions.Exception'>,)),
    [(<type 'exceptions.EnvironmentError'>,
      (<type 'exceptions.StandardError'>,)),
     [(<type 'exceptions.OSError'>,
       (<type 'exceptions.EnvironmentError'>,))]]]]]]

object()
BaseException(object)
Exception(BaseException)
StandardError(Exception)
EnvironmentError(StandardError)
OSError(EnvironmentError)