gh-160913: Use the dict freelist in dict() (#160922)

Use the dict freelist in dict()

dict_vectorcall() allocated every instance with _PyType_AllocNoTrack(),
so dict() never reused the objects that {} and PyDict_New() recycle
through the freelist. Allocate with PyDict_New() instead. tp_vectorcall
is not inherited by subclasses, so dict_vectorcall() only ever runs for
the exact dict type; assert that rather than checking it.

Co-authored-by: Claude Fable 5.1 <noreply@anthropic.com>

Author: eendebakpt

Objects/dictobject.c

@@ -5415,7 +5415,8 @@ dict_vectorcall(PyObject *type, PyObject * const*args,
         return NULL;
     }
 
-    PyObject *self = dict_new(_PyType_CAST(type), NULL, NULL);
+    assert(type == (PyObject *)&PyDict_Type);
+    PyObject *self = PyDict_New();
     if (self == NULL) {
         return NULL;
     }