The only (ugly) workaround on Python 3+ is to encode string to bytes first:
dben.put(bytes(str(3), "ascii"), bytes(str(2), "ascii"))
or, more conveniently:
dben.put(str(3).encode("ascii"), str(2).encode("ascii"))
>>> dben.exists(bytes(2, "ascii"))
False
>>> dben.exists(bytes(3, "ascii"))
True