WIP: Revert "XMLExportImport: don't support bytes for now"

This reverts commit 3145b32f.

Otherwise, ERP5 site setup fails while importing erp5_core/ToolTemplateItem/portal_transforms.xml.

/SR/parts/erp5/product/ERP5/Document/BusinessTemplate.py(930)_importFile()
    928         # FIXME: Why not use the importXML function directly? Are there any BT5s
    929         # with actual .zexp files on the wild?
--> 930         obj = connection.importFile(file_obj, customImporters=customImporters)
    931       self._objects[obj_key] = obj
    932

ipdb> file_obj
<_io.BufferedReader name='/SR/parts/erp5/product/ERP5/bootstrap/erp5_core/ToolTemplateItem/portal_transforms.xml'>
...
> /SR/develop-eggs/zodbpickle-2.0.0+slapospatched001-py3.8-linux-x86_64.egg/zodbpickle/pickle_3.py(995)decode_string()
    993                 return value
    994         else:
--> 995             return value.decode(self.encoding, self.errors)
    996
    997     def load_string(self):

ipdb> p value
b'\x00\x00\x00\x00\x00\x00\x00\x80'