bpo-18372: Add missing PyObject_GC_Track() calls in the pickle module (GH-8505)

Misc/NEWS.d/next/Core and Builtins/2018-12-08-03-40-43.bpo-18372.DT1nR0.rst

+Add missing :c:func:`PyObject_GC_Track` calls in the :mod:`pickle` module.
+Patch by Zackery Spytz.

Modules/_pickle.c

@@ -1119,6 +1119,8 @@ _Pickler_New(void)
         Py_DECREF(self);
         return NULL;
     }
+
+    PyObject_GC_Track(self);
     return self;
 }
 
@@ -1496,6 +1498,7 @@ _Unpickler_New(void)
         return NULL;
     }
 
+    PyObject_GC_Track(self);
     return self;
 }