Fix Binary on python < 2.7.4 (closes #10593811)
Before the fix for http://bugs.python.org/issue10211 we can't pass a
buffer object to BytesIO, so keep using StringIO in that case. And
because StringIO is an old-style class, we can't use super().
usr/lib/python2*/*-packages/cubicweb/entities/
usr/lib/python2*/*-packages/cubicweb/ext/
usr/share/cubicweb/cubes/
usr/lib/python2*/*-packages/cubicweb/*.py