[Autotest][PY3] Migrating server/hosts/drone_api_client

Migrating server/hosts/drone_api_client/* to python3.
This code is not *yet* used, so there is little to no risk of any
possible breakages :)

BUG=chromium:990593
TEST=py_compile in python 2 and 3.

Change-Id: I856ff04e91dd4b04deb95ab8583dba2a90c396b7
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/third_party/autotest/+/2492800
Tested-by: Derek Beckett <dbeckett@chromium.org>
Auto-Submit: Derek Beckett <dbeckett@chromium.org>
Reviewed-by: Greg Edelston <gredelston@google.com>
Commit-Queue: Greg Edelston <gredelston@google.com>
diff --git a/server/hosts/drone_api_client/client.py b/server/hosts/drone_api_client/client.py
index b052a5b..5d360f3 100644
--- a/server/hosts/drone_api_client/client.py
+++ b/server/hosts/drone_api_client/client.py
@@ -1,9 +1,10 @@
+# Lint as: python2, python3
 """Client for Autotest side communcations to the TLS SSH Server."""
 
 
-import cStringIO
 import grpc
 import logging
+import six
 import time
 
 import common_pb2
@@ -62,8 +63,8 @@
         start_time = time.time()
         response = self._send_cmd(cmd, timeout)
 
-        stdout_buf = cStringIO.StringIO()
-        stderr_buf = cStringIO.StringIO()
+        stdout_buf = six.StringIO()
+        stderr_buf = six.StringIO()
         last_status = 0
 
         if response: