Ensure that PySet_Add() operates on a newly created frozenset, like PyTuple_SetItem does.
Add PyFrozenSet_Check(), which was not needed before; The list of Py*Set_Check* macros seems to be complete now.
Add missing NEWS entries about all this.
diff --git a/Misc/NEWS b/Misc/NEWS
index 8dacce0..6291729 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -1471,6 +1471,12 @@
C API
-----
+- ``PySet_Add()`` can now modify a newly created frozenset. Similarly to
+ ``PyTuple_SetItem``, it can be used to populate a brand new frozenset; but
+ it does not steal a reference to the added item.
+
+- Added ``PySet_Check()`` and ``PyFrozenSet_Check()`` to the set API.
+
- Backport of PyUnicode_FromString(), _FromStringAndSize(), _Format and
_FormatV from Python 3.0. Made PyLong_AsSsize_t and PyLong_FromSsize_t
public functions.