Use BytesIO instead of StringIO. Use text_type instead of unicode.

Signed-off-by: Roberto Rosario <roberto.rosario.gonzalez@gmail.com>

mayan/apps/common/compressed_files.py

@@ -1,5 +1,6 @@
 from __future__ import unicode_literals
 
+from io import BytesIO
 import zipfile
 
 try:
@@ -8,11 +9,6 @@ try:
 except:
     COMPRESSION = zipfile.ZIP_STORED
 
-try:
-    from cStringIO import StringIO
-except ImportError:
-    from StringIO import StringIO
-
 from django.core.files.uploadedfile import SimpleUploadedFile
 
 
@@ -35,7 +31,7 @@ class CompressedFile(object):
             self._create()
 
     def _create(self):
-        self.descriptor = StringIO()
+        self.descriptor = BytesIO()
         self.zf = zipfile.ZipFile(self.descriptor, mode='w')
 
     def _open(self, file_input):