gh-106320: Remove private _PyDict C API (#107145)

Move private _PyDict functions to the internal C API (pycore_dict.h):

* _PyDict_Contains_KnownHash()
* _PyDict_DebugMallocStats()
* _PyDict_DelItemIf()
* _PyDict_GetItemWithError()
* _PyDict_HasOnlyStringKeys()
* _PyDict_MaybeUntrack()
* _PyDict_MergeEx()

No longer export these functions.

Objects/setobject.c

@@ -32,6 +32,7 @@
 */
 
 #include "Python.h"
+#include "pycore_dict.h"          // _PyDict_Contains_KnownHash()
 #include "pycore_modsupport.h"    // _PyArg_NoKwnames()
 #include "pycore_object.h"        // _PyObject_GC_UNTRACK()
 #include "pycore_setobject.h"     // _PySet_NextEntry() definition