factory: Use ''.join instead of string.join()

`string.join()` is deprecated in python3.

BUG=chromium:999876
TEST=make test
TEST=make lint

Change-Id: I980986db62d83847672d2cf9b61d14bd532d7d71
Reviewed-on: https://chromium-review.googlesource.com/1894775
Tested-by: Yilin Yang <kerker@chromium.org>
Commit-Ready: Yilin Yang <kerker@chromium.org>
Legacy-Commit-Queue: Commit Bot <commit-bot@chromium.org>
Reviewed-by: Cheng-Han Yang <chenghan@chromium.org>
diff --git a/devtools/instalog/get_attachments.py b/devtools/instalog/get_attachments.py
index 8ca122f..1e02e00 100755
--- a/devtools/instalog/get_attachments.py
+++ b/devtools/instalog/get_attachments.py
@@ -16,7 +16,6 @@
 import json
 import os
 import shutil
-import string
 import subprocess
 import sys
 
@@ -68,7 +67,7 @@
       '    attachment.key LIKE "%%%s%%" AND',
       '    time >= TIMESTAMP("%s") AND',
       '    time < TIMESTAMP("%s")']
-  query_statement = string.join(query_statement_words, '\n')
+  query_statement = '\n'.join(query_statement_words)
   query = query_statement % (
       DATE_FORMAT, args.serial_number_key, PROJECT_ID, args.dataset_id,
       args.attachment_key, args.start_date, args.end_date)