Don't rely on c.name when looking for constraint names. (#744)

* Don't rely on `c.name` when looking for constraint names.

`c.name` may be `None` while the constraint at least has an
auto-generated name as key in `lp.constraints()`.

* Fix formatting.

pulp/apis/xpress_api.py

@@ -471,11 +471,11 @@ def findSolutionValues(self, lp):
                 lp.assignVarsDj({v.name: djs[v._xprs[0]] for v in lp.variables()})
             if duals is not None:
                 lp.assignConsPi(
-                    {c.name: duals[c._xprs[0]] for c in lp.constraints.values()}
+                    {n: duals[c._xprs[0]] for (n, c) in lp.constraints.items()}
                 )
             if slacks is not None:
                 lp.assignConsSlack(
-                    {c.name: slacks[c._xprs[0]] for c in lp.constraints.values()}
+                    {n: slacks[c._xprs[0]] for (n, c) in lp.constraints.items()}
                 )
 
             status = statusmap.get(