Use PyFrozenDict_Check and PyFrozenDict_CheckExact

Found out these exist while reading through
https://vstinner.github.io/pep-814-add-frozendict-builtin-type.html.

https://github.com/python/cpython/blob/39a9a47516abf8a68cb3239f968c2b2135c3a5a7/Include/cpython/dictobject.h#L35-L43

Signed-off-by: Edgar Ramírez Mondragón <edgarrm358@gmail.com>

Author: edgarrmondragon

msgpack/_packer.pyx

@@ -1,5 +1,3 @@
-import sys
-
 from cpython cimport *
 from cpython.bytearray cimport PyByteArray_Check, PyByteArray_CheckExact
 from cpython.datetime cimport (
@@ -12,21 +10,16 @@ cdef Timestamp
 
 from .ext import ExtType, Timestamp
 
-# frozendict is a builtin type added in Python 3.15 (PEP 814).
-if sys.version_info >= (3, 15):
-    import builtins
-
-    frozendict = builtins.frozendict
-else:
-
-    class frozendict:
-        pass
-
 
 cdef extern from "Python.h":
 
     int PyMemoryView_Check(object obj)
 
+cdef extern from "frozendict_compat.h":
+
+    bint PyFrozenDict_Check(object obj)
+    bint PyFrozenDict_CheckExact(object obj)
+
 cdef extern from "pack.h":
     struct msgpack_packer:
         char* buf
@@ -214,8 +207,8 @@ cdef class Packer:
                 rawval = o
             msgpack_pack_raw(&self.pk, L)
             msgpack_pack_raw_body(&self.pk, rawval, L)
-        elif (PyDict_CheckExact(o) or type(o) is frozendict) if strict \
-                else (PyDict_Check(o) or isinstance(o, frozendict)):
+        elif (PyDict_CheckExact(o) or PyFrozenDict_CheckExact(o)) if strict \
+                else (PyDict_Check(o) or PyFrozenDict_Check(o)):
             L = len(o)
             if L > ITEM_LIMIT:
                 raise ValueError("dict is too large")

msgpack/frozendict_compat.h

@@ -0,0 +1,15 @@
+#ifndef MSGPACK_FROZENDICT_COMPAT_H
+#define MSGPACK_FROZENDICT_COMPAT_H
+
+#include <Python.h>
+
+/* frozendict (PEP 814) is a builtin type added in CPython 3.15. */
+#if PY_VERSION_HEX >= 0x030f0000
+/* PyFrozenDict_Check / PyFrozenDict_CheckExact come from
+ * Include/cpython/dictobject.h, pulled in transitively by Python.h. */
+#else
+#define PyFrozenDict_Check(op) 0
+#define PyFrozenDict_CheckExact(op) 0
+#endif
+
+#endif