Make it clear this is abstract class

Signed-off-by: Michal Čihař <michal@cihar.com>

weblate/trans/exporters.py

@@ -20,7 +20,6 @@
 """Exporter using translate-toolkit"""
 from __future__ import unicode_literals
 
-from io import BytesIO
 import string
 
 from django.http import HttpResponse
@@ -185,6 +184,9 @@ class XMLExporter(BaseExporter):
         else:
             return text.translate(_CHARMAP)
 
+    def get_storage(self):
+        raise NotImplementedError()
+
 
 @register_exporter
 class PoXliffExporter(XMLExporter):