bpo-45316: Move private PyDict functions to internal C API (GH-31577)

Move the following private unexported functions to the internal C API
headers:

* _PyDictKeys_GetVersionForCurrentState()
* _PyDictKeys_StringLookup()
* _PyDict_FromKeys()
* _PyDict_GetItemHint()
* _PyDict_KeysSize()
* _PyDict_LoadGlobal()
* _PyDict_NewKeysForClass()
* _PyDict_Pop_KnownHash()
* _PyDict_SetItem_Take2()
* _PyObjectDict_SetItem()
* _PyObject_MakeDictFromInstanceAttributes()
* _Py_dict_lookup()