Removed polyglots unicode_type usage

ebook_converter/ebooks/mobi/reader/mobi6.py

@@ -16,7 +16,7 @@ from ebook_converter.ebooks.metadata.toc import TOC
 from ebook_converter.ebooks.mobi.reader.headers import BookHeader
 from ebook_converter.utils.img import save_cover_data_to, gif_data_to_png_data, AnimatedGIF
 from ebook_converter.utils.imghdr import what
-from ebook_converter.polyglot.builtins import iteritems, unicode_type
+from ebook_converter.polyglot.builtins import iteritems
 
 
 __license__ = 'GPL v3'
@@ -287,7 +287,7 @@ class MobiReader(object):
             pass
 
         def write_as_utf8(path, data):
-            if isinstance(data, unicode_type):
+            if isinstance(data, str):
                 data = data.encode('utf-8')
             with lopen(path, 'wb') as f:
                 f.write(data)