Add a new Py_TPFLAGS_DISALLOW_INSTANTIATION type flag to disallow
creating type instances: set tp_new to NULL and don't create the
"__new__" key in the type dictionary.

The flag is set automatically on static types if tp_base is NULL or
&PyBaseObject_Type and tp_new is NULL.

Use the flag on the following types:

* _curses.ncurses_version type
* _curses_panel.panel
* _tkinter.Tcl_Obj
* _tkinter.tkapp
* _tkinter.tktimertoken
* _xxsubinterpretersmodule.ChannelID
* sys.flags type
* sys.getwindowsversion() type
* sys.version_info type

Update MyStr example in the C API documentation to use
Py_TPFLAGS_DISALLOW_INSTANTIATION.

Add _PyStructSequence_InitType() function to create a structseq type
with the Py_TPFLAGS_DISALLOW_INSTANTIATION flag set.

type_new() calls _PyType_CheckConsistency() at exit.

Reorganize pycore_interp_init() to initialize singletons before the
the first PyType_Ready() call. Fix an issue when Python is configured
using --without-doc-strings.

No longer use deprecated aliases to functions:

* Replace PyMem_MALLOC() with PyMem_Malloc()
* Replace PyMem_REALLOC() with PyMem_Realloc()
* Replace PyMem_FREE() with PyMem_Free()
* Replace PyMem_Del() with PyMem_Free()
* Replace PyMem_DEL() with PyMem_Free()

Modify also the PyMem_DEL() macro to use directly PyMem_Free().

These functions are considered not safe because they suppress all internal errors
and can return wrong result.  PyDict_GetItemString and _PyDict_GetItemId can
also silence current exception in rare cases.

Remove no longer used _PyDict_GetItemId.
Add _PyDict_ContainsId and rename _PyDict_Contains into
_PyDict_Contains_KnownHash.

Use _PyType_HasFeature() in the _io module and in structseq
implementation. Replace PyType_HasFeature() opaque function call with
_PyType_HasFeature() inlined function.