bpo-43916: Add Py_TPFLAGS_DISALLOW_INSTANTIATION type flag (GH-25721)

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.

1 files changed, 3 insertions, 2 deletions

diff --git a/Lib/test/test_sys.py b/Lib/test/test_sys.py
index ee39375af3..1fd5247a91 100644
--- a/Lib/test/test_sys.py
+++ b/Lib/test/test_sys.py
@@ -605,11 +605,12 @@ class SysModuleTest(unittest.TestCase):
     def assert_raise_on_new_sys_type(self, sys_attr):
         # Users are intentionally prevented from creating new instances of
         # sys.flags, sys.version_info, and sys.getwindowsversion.
+        arg = sys_attr
         attr_type = type(sys_attr)
         with self.assertRaises(TypeError):
-            attr_type()
+            attr_type(arg)
         with self.assertRaises(TypeError):
-            attr_type.__new__(attr_type)
+            attr_type.__new__(attr_type, arg)
 
     def test_sys_flags_no_instantiation(self):
         self.assert_raise_on_new_sys_type(sys.flags)