global: StringIO from six
global: StringIO from six

Signed-off-by: Dimitrios Semitsoglou-Tsiapos <6a6862363aaa6e93bbf0fa721048d2ca97773c19@cern.ch>

inveniosoftware/invenio-base,tiborsimko/invenio-base,inveniosoftware/invenio-base,tiborsimko/invenio-base