Replace unicode with six.text_type

from xml.sax.saxutils import XMLGenerator

from xml.sax.xmlreader import AttributesImpl

import six

from pybtex.database.output import BaseWriter

        >>> from pybtex.database import BibliographyData

        >>> data = BibliographyData()

        >>> unicode_xml = Writer().to_string(data)

        >>> isinstance(unicode_xml, unicode)

        >>> isinstance(unicode_xml, six.text_type)

        True

        >>> print(unicode_xml)

        <bibtex:file xmlns:bibtex="http://bibtexml.sf.net/">