How come string.maketrans does not work in Python 3.1?

I'm a Python newbie.

How come this doesn't work in Python 3.1?

from string import maketrans   # Required to call maketrans function.

intab = "aeiou"
outtab = "12345"
trantab = maketrans(intab, outtab)

str = "this is string example....wow!!!";
print str.translate(trantab);

When I executed the above code, I get the following instead:

Traceback (most recent call last):
  File "<pyshell#119>", line 1, in <module>
    transtab = maketrans(intab, outtab)
  File "/Library/Frameworks/Python.framework/Versions/3.1/lib/python3.1/string.py", line 60, in maketrans
    raise TypeError("maketrans arguments must be bytes objects")
TypeError: maketrans arguments must be bytes objects

What does "must be bytes objects" mean? Could anyone please help post a working code for Python 3.1 if it's possible?