Import unquote_plus fro django.six.

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

mayan/apps/metadata/api.py

@@ -1,8 +1,7 @@
 from __future__ import unicode_literals
 
-from urllib import unquote_plus
-
 from django.shortcuts import get_object_or_404
+from django.utils.six.moves.urllib.parse import unquote_plus
 
 from .models import DocumentMetadata, MetadataType