Cython: minor fixes

python/obitools3/dms/taxo/taxo.pyx

@@ -110,7 +110,7 @@ cdef class Taxonomy(OBIWrapper) :
         if type(ref) == int :
             return self.get_taxon_by_taxid(ref)
         else :
-            raise Exception("Not implemented")
+            raise NotImplementedError()
 
 
     cpdef Taxon get_taxon_by_taxid(self, int taxid):   # TODO check deleted taxon behavior (403122)