$ python test123.py 
Traceback (most recent call last):
  File "test123.py", line 9, in <module>
    ldif_writer.unparse(dn, entry)
  File "/usr/lib/python2.6/site-packages/ldif.py", line 195, in unparse
    self._unparseAttrTypeandValue('dn',dn)
  File "/usr/lib/python2.6/site-packages/ldif.py", line 142, in _unparseAttrTypeandValue
    self._unfoldLDIFLine(': '.join([attr_type,attr_value]))
  File "/usr/lib/python2.6/site-packages/ldif.py", line 107, in _unfoldLDIFLine
    self._output_file.write(line)
  File "/usr/lib/python2.6/io.py", line 1500, in write
    s.__class__.__name__)
TypeError: can't write str to text stream

$ cat test123.py 
# coding: utf-8

import io, ldif

o = io.StringIO()
ldif_writer = ldif.LDIFWriter(o)
dn='cn=Michael Stroeder,ou=Test'
entry={'objectClass':['top','person'],'cn':['Michael Stroeder'],'sn':['Stroeder']}
ldif_writer.unparse(dn, entry)

# coding: utf-8

import io, ldif

class StrIO(io.StringIO):
    def write(self, a):
        return io.StringIO.write(self, unicode(a))

o = StrIO()

ldif_writer = ldif.LDIFWriter(o)
dn='cn=Michael Stroeder,ou=Test'
entry={'objectClass':['top','person'],'cn':['Michael Stroeder'],'sn':['Stroeder']}
ldif_writer.unparse(dn, entry)