Bug#1122009: python3-msgpack gets silently misbuilt with Cython 3.1

[Adrian Bunk]

Package: python3-msgpack
Version: 1.0.3-3
Severity: grave

$ debdiff python3-msgpack_1.0.3-3+b4_amd64.deb  
python3-msgpack_1.0.3-3+b5_amd64.deb
...
Files in first .deb but not in second
-------------------------------------
-rw-r--r--  root/root   
/usr/lib/python3/dist-packages/msgpack/_cmsgpack.cpython-313-x86_64-linux-gnu.so
...

Due to:

https://buildd.debian.org/status/fetch.php?pkg=python-msgpack&arch=amd64&ver=1.0.3-3%2Bb5&stamp=1764951672&raw=0

...
cythonize: 'msgpack/_cmsgpack.pyx'

Error compiling Cython file:
------------------------------------------------------------
...
                        o = self._default(o)
                        default_used = True
                        continue
                    else:
                        raise OverflowError("Integer value out of range")
            elif PyInt_CheckExact(o) if strict_types else PyInt_Check(o):
                 ^
------------------------------------------------------------

msgpack/_packer.pyx:189:17: undeclared name not builtin: PyInt_CheckExact

Error compiling Cython file:
------------------------------------------------------------
...
                        o = self._default(o)
                        default_used = True
                        continue
                    else:
                        raise OverflowError("Integer value out of range")
            elif PyInt_CheckExact(o) if strict_types else PyInt_Check(o):
                                                          ^
------------------------------------------------------------

msgpack/_packer.pyx:189:58: undeclared name not builtin: PyInt_Check
...