Avoid calling fails_species_constraints() twice in a row.

Since Python 3.8 we can do this "walrus" operator :=

Author: rwest

rmgpy/data/kinetics/family.py

@@ -1655,8 +1655,7 @@ def _generate_product_structures(self, reactant_structures, maps, forward, relab
         for struct in product_structures:
             if self.is_molecule_forbidden(struct):
                 raise ForbiddenStructureException()
-            if fails_species_constraints(struct):
-                reason = fails_species_constraints(struct)
+            if (reason := fails_species_constraints(struct)):
                 raise ForbiddenStructureException(
                     "Species constraints forbids product species {0}. Please "
                     "reformulate constraints, or explicitly "