bpo-26163: Frozenset hash improvement (#5194)

Misc/NEWS.d/next/Core and Builtins/2018-01-14-20-32-47.bpo-26163.xv9Iuv.rst

+Improved frozenset() hash to create more distinct hash values when faced
+with datasets containing many similar values.

Objects/setobject.c

@@ -795,6 +795,7 @@ frozenset_hash(PyObject *self)
     hash ^= ((Py_uhash_t)PySet_GET_SIZE(self) + 1) * 1927868237UL;
 
     /* Disperse patterns arising in nested frozensets */
+    hash ^= (hash >> 11) ^ (~hash >> 25);
     hash = hash * 69069U + 907133923UL;
 
     /* -1 is reserved as an error code */