create cache directory, if not exists

syng/sources/s3.py

@@ -94,6 +94,8 @@ class S3Source(FileBasedSource):
                 if obj.object_name is not None and self.has_correct_extension(obj.object_name)
             ]
             if self.index_file is not None and not os.path.isfile(self.index_file):
+                os.makedirs(os.path.dirname(self.index_file), exist_ok=True)
+
                 with open(self.index_file, "w", encoding="utf8") as index_file_handle:
                     dump(file_list, index_file_handle)
             return file_list