Use context manager to avoid open files

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

mayan/apps/documents/models/document_version_models.py

@@ -175,7 +175,8 @@ class DocumentVersion(models.Model):
             logger.debug('Intermidiate file "%s" not found.', cache_filename)
 
             try:
-                converter = get_converter_class()(file_object=self.open())
+                with self.open() as version_file_object:
+                    converter = get_converter_class()(file_object=version_file_object)
                     with converter.to_pdf() as pdf_file_object:
 
                         # Since open "wb+" doesn't create files, check if the file