Use six.StringIO/BytesIO instead of StringIO.StringIO

to keep Python 3.x compatibility, use six.StringIO/BytesIO to
replace StringIO.StringIO

StringIO works for unicode
BytesIO works for bytes

Change-Id: I063d62a4f005084f4088da403f348ad97656ad45
Partial-Bug: #1280100
diff --git a/tools/find_stack_traces.py b/tools/find_stack_traces.py
index 4862d01..49a42fe 100755
--- a/tools/find_stack_traces.py
+++ b/tools/find_stack_traces.py
@@ -18,7 +18,7 @@
 import gzip
 import pprint
 import re
-import StringIO
+import six
 import sys
 import urllib2
 
@@ -68,7 +68,7 @@
     req = urllib2.Request(url)
     req.add_header('Accept-Encoding', 'gzip')
     page = urllib2.urlopen(req)
-    buf = StringIO.StringIO(page.read())
+    buf = six.StringIO(page.read())
     f = gzip.GzipFile(fileobj=buf)
     content = f.read()