Fix `po_import` behavior in Python 2

See merge request !1929
4 jobs for zope4py3 in 0 seconds
Status Job ID Name Coverage
  External
failed ERP5.CodingStyleTest-Zope4Py3

00:47:36

passed ERP5.CodingStyleTest-Zope4Py3-py2

00:47:04

failed ERP5.UnitTest-Zope4Py3

03:56:58

failed ERP5.UnitTest-Zope4Py3-Py2

02:28:08