Mark raw strings as such

This is raising warnings now and will be an error in future versions of
Python 3.

Change-Id: I8827cb36ef2ef85e8f245a2a181fb23ee75bfd16
diff --git a/tools/check_logs.py b/tools/check_logs.py
index b80ccc0..de7e41d 100755
--- a/tools/check_logs.py
+++ b/tools/check_logs.py
@@ -96,7 +96,7 @@
 def collect_url_logs(url):
     page = urlreq.urlopen(url)
     content = page.read()
-    logs = re.findall('(screen-[\w-]+\.txt\.gz)</a>', content)
+    logs = re.findall(r'(screen-[\w-]+\.txt\.gz)</a>', content)
     return logs
 
 
@@ -162,6 +162,7 @@
     print("ok")
     return 0
 
+
 usage = """
 Find non-white-listed log errors in log files from a devstack-gate run.
 Log files will be searched for ERROR or CRITICAL messages. If any