gh-106320: Remove private _PyObject C API (#107147)

Move private debug _PyObject functions to the internal C API
(pycore_object.h):

* _PyDebugAllocatorStats()
* _PyObject_CheckConsistency()
* _PyObject_DebugTypeStats()
* _PyObject_IsFreed()

No longer export most of these functions, except of
_PyObject_IsFreed().

Move test functions using _PyObject_IsFreed() from _testcapi to
_testinternalcapi. check_pyobject_is_freed() test no longer catch
_testcapi.error: the tested function cannot raise _testcapi.error.