Don't track multidict if it contains untrackable objects only

[asvetlov]

Multidict is used mostly for storing primitive types as keys and values: str, int, float, None.

PyObject_GC_Track should be called only if a key or value doesn't belong to a primitive type.

BTW, now PyObject_GC_Track is not called for multidicts and proxies which is an error.

[asvetlov]

Sorry, I was wrong.
All MultiDict/CIMultiDict/MultiDictProxy/CIMultiDictProxy use PyType_GenericNew and PyType_GenericAlloc for tp_new/tp_alloc slots.
PyType_GenericAlloc automatically tracks GC if Py_TPFLAGS_HAVE_GC flag is set.

Explicit managing still makes sense as a microoptimization.