Partially revert commit 1a6be91e,
move back PyGC API from the internal API to the C API:

* _PyGCHead_NEXT(g), _PyGCHead_SET_NEXT(g, p)
* _PyGCHead_PREV(g), _PyGCHead_SET_PREV(g, p)
* _PyGCHead_FINALIZED(g), _PyGCHead_SET_FINALIZED(g)
* _PyGC_FINALIZED(o), _PyGC_SET_FINALIZED(o)
* _PyGC_PREV_MASK_FINALIZED
* _PyGC_PREV_MASK_COLLECTING
* _PyGC_PREV_SHIFT
* _PyGC_PREV_MASK

_PyObject_GC_TRACK(o) and _PyObject_GC_UNTRACK(o) remain in the
internal API.

Fixes assertion failures in _datetimemodule.c
introduced in the previous fix (see bpo-31752).

Rather of trying to handle an int subclass as exact int,
let it to use overridden special methods, but check the
result of divmod().

locale.localeconv() now sets temporarily the LC_CTYPE locale to the
LC_MONETARY locale if the two locales are different and monetary
strings are non-ASCII. This temporary change affects other threads.

Changes:

* locale.localeconv() can now set LC_CTYPE to LC_MONETARY to decode
  monetary fields.
* Add LocaleInfo.grouping_buffer: copy localeconv() grouping string
  since it can be replaced anytime if a different thread calls
  localeconv().
* _Py_GetLocaleconvNumeric() now requires a "struct lconv *"
  structure, so locale.localeconv() now longer calls localeconv()
  twice. Moreover, the function now requires all arguments to be
  non-NULL.
* Rename STATIC_LOCALE_INFO_INIT to LocaleInfo_STATIC_INIT.
* Move _Py_GetLocaleconvNumeric() definition from fileutils.h
  to pycore_fileutils.h. pycore_fileutils.h now includes locale.h.
* The _locale module is now built with Py_BUILD_CORE defined.

coro->cr_origin wasn't initialized if compute_cr_origin() failed in
PyCoro_New(), which would cause a crash during the coroutine's
deallocation.



https://bugs.python.org/issue35269

Source of T_STRING: https://github.com/python/cpython/blob/e42b705188271da108de42b55d9344642170aa2b/Python/structmember.c#L51

Source of PyUnicode_FromString
https://github.com/python/cpython/blob/master/Include/unicodeobject.h#L702


https://bugs.python.org/issue25438

[Issue 28401](https://bugs.python.org/issue28401): Don't attempt to import the stable API extensions, they are not supported in PyDEBUG builds (which don't implement that ABI).


https://bugs.python.org/issue28401

This missed PyErr_NoMemory() could cause a SystemError when calling
_symtable.symtable().

* The _PySys_EndInit() function now copies the
  config->module_search_path list, so config is longer modified when
  sys.path is updated.
* config->warnoptions list and config->xoptions dict are also copied
* test_embed: InitConfigTests now also tests
  main_config['module_search_path']
* Fix _Py_InitializeMainInterpreter(): don't use config->warnoptions
   but sys.warnoptions to decide if the warnings module should
   be imported at startup.

Remove an unnecessary "that":
... will execute that the body ... ->  ... will execute the body ...

PyTuple_Pack can fail and return NULL. If this happens, then PyType_FromSpecWithBases will incorrectly create a new type without bases. Also, it will crash on the Py_DECREF that follows. Also free members and type in error conditions.

"single" needs to be decrefed if PyList_Append() fails.

test_embed.InitConfigTests tests more configuration variables.

Changes:

* InitConfigTests tests more core configuration variables:

  * base_exec_prefix
  * base_prefix
  * exec_prefix
  * home
  * legacy_windows_fs_encoding
  * legacy_windows_stdio
  * module_search_path_env
  * prefix

* "_testembed init_from_config" tests more variables:

  * argv
  * warnoptions
  * xoptions

* InitConfigTests: add check_global_config(), check_core_config() and
  check_main_config() subfunctions to cleanup the code. Move also
  constants at the class level (ex: COPY_MAIN_CONFIG).
* Fix _PyCoreConfig_AsDict(): don't set stdio_encoding twice
* Use more macros in _PyCoreConfig_AsDict() and
  _PyMainInterpreterConfig_AsDict() to reduce code duplication.
* Other minor cleanups.

Remove _Py_REF_DEBUG_COMMA, DK_DEBUG_INCREF, and DK_DEBUG_DECREF.
Convert DK_INCREF and DK_DECREF to static inline functions.

* Fix _PyCoreConfig_SetGlobalConfig(): set also Py_FrozenFlag
* Fix _PyCoreConfig_AsDict(): export also xoptions
* Add _Py_GetGlobalVariablesAsDict() and _testcapi.get_global_config()
* test.pythoninfo: dump also global configuration variables
* _testembed now serializes global, core and main configurations
  using JSON to reuse _Py_GetGlobalVariablesAsDict(),
  _PyCoreConfig_AsDict() and _PyMainInterpreterConfig_AsDict(),
  rather than duplicating code.
* test_embed.InitConfigTests now test much more configuration
  variables

Discovered using clang's MemorySanitizer when it ran python3's
test_fstring test_misformed_unicode_character_name.

An msan build will fail by simply executing: ./python -c 'u"\N"'

* Fix _PyMainInterpreterConfig_Copy():
   copy 'install_signal_handlers' attribute
* Add _PyMainInterpreterConfig_AsDict()
* Add unit tests on the main interpreter configuration
  to test_embed.InitConfigTests
* test.pythoninfo: log also main_config

If tracemalloc is not tracing Python memory allocations,
_PyMem_DumpTraceback() now suggests to enable tracemalloc
to get the traceback where the memory block has been allocated.

* Move "GC" macros together:

  * PyObject_IS_GC()
  * _Py_AS_GC()
  * _PyObject_GC_IS_TRACKED()
  * _PyObject_GC_MAY_BE_TRACKED()

* Mark other GC macros as internal (#ifdef Py_BUILD_CORE):

  * _PyGCHead_NEXT(g), _PyGCHead_SET_NEXT(g, p)
  * _PyGCHead_PREV(g), _PyGCHead_SET_PREV(g, p)
  * _PyGCHead_FINALIZED(g), _PyGCHead_SET_FINALIZED(g)
  * _PyGC_FINALIZED(o), _PyGC_SET_FINALIZED(o)
  * _PyObject_GC_TRACK(o), _PyObject_GC_UNTRACK(o)
  * _PyGC_PREV_MASK_FINALIZED
  * _PyGC_PREV_MASK_COLLECTING
  * _PyGC_PREV_SHIFT
  * _PyGC_PREV_MASK

* Replace _PyGC_generation0 with _PyRuntime.gc.generation0
* _queuemodule.c: replace _PyObject_GC_UNTRACK()
  with with PyObject_GC_UnTrack()
* Document that  _PyObject_GC_TRACK() _PyObject_GC_UNTRACK() macros
  have been removed from the public C API.