bpo-40989: PyObject_INIT() becomes an alias to PyObject_Init() (GH-20901)

The PyObject_INIT() and PyObject_INIT_VAR() macros become aliases to,
respectively, PyObject_Init() and PyObject_InitVar() functions.

Rename _PyObject_INIT() and _PyObject_INIT_VAR() static inline
functions to, respectively, _PyObject_Init() and _PyObject_InitVar(),
and move them to pycore_object.h. Remove their return value:
their return type becomes void.

The _datetime module is now built with the Py_BUILD_CORE_MODULE macro
defined.

Remove an outdated comment on _Py_tracemalloc_config.