task.tests: Fix StringIO import
task.tests: Fix StringIO import

It's in the io module now.

Signed-off-by: Zack Cerza <d7cdf09fc0f0426e98c9978ee42da5d61fa54986@redhat.com>

ceph/teuthology,ktdreyer/teuthology,ceph/teuthology,ktdreyer/teuthology