PortalTransforms: update transforms/rest for zope4 compatibility

Update to https://github.com/plone/Products.PortalTransforms/blob/3.2.0/Products/PortalTransforms/transforms/rest.py
because Zope4 no longer ship with a reStructuredText module and recommend
using docutils directly instead, which is what upstream did.

( in zope4py2 branch this was working because slapos patch
component/egg-patch/Zope/0001-OFS-XMLExportImport.patch used to bring
back reStructuredText but we don't seem to need it )