docker: docker.py wrap StringIO import for python3

Although the docker.py is nominally python2 we actually invoke it with
the configured python from the configure script.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>

tests/docker/docker.py

@@ -26,7 +26,10 @@ import tempfile
 import re
 import signal
 from tarfile import TarFile, TarInfo
-from StringIO import StringIO
+try:
+    from StringIO import StringIO
+except ImportError:
+    from io import StringIO
 from shutil import copy, rmtree
 from pwd import getpwuid