ERP5OOo: fix office:include_img on python3

We were applying str() to PData or bytes, resulting on broken image
inserted, something like "b'PNG'" instead of the expected "PNG"
7 jobs for zope4py3 in 0 seconds
Status Job ID Name Coverage
  External
passed ERP5.CodingStyleTest-Zope4Py3

01:34:59

passed ERP5.CodingStyleTest-Zope4Py3-py2

01:29:57

failed ERP5.UnitTest-Zope4Py3

03:43:32

passed ERP5.UnitTest-Zope4Py3-Py2

01:49:48

passed ERP5.CodingStyleTest-Zope4Py3

01:42:50

canceled ERP5.UnitTest-Zope4Py3

failed ERP5.UnitTest-Zope4Py3

02:53:00