[sobjects] fix fresh.workcases importer

In extentities generator, extid is cwuri from database which is a text field.
So in python3 exteid2eid[extid] wasn't matching anything because extid was bytes.
Use text extid instead.

cubicweb_fresh/sobjects.py

@@ -52,7 +52,6 @@
                                 ' X cw_source S, S eid %(s)s'.format(etype),
                                 {'s': self.source.eid})
         for extid, eid in rset:
-            extid = extid.encode('ascii')
             self._source_uris[extid] = (eid, etype)
             yield extid, eid