$ ./python -m test test_multibytecodec -m test.test_multibytecodec.Test_IncrementalEncoder.test_subinterp -R 3:3
(...)
test_multibytecodec leaked [258, 258, 258] references, sum=774

I simplified the code. The following test leaks references:

    def test_subinterp(self):
        import _testcapi
        code = textwrap.dedent("""
            import _codecs_jp
            codec = _codecs_jp.getcodec('cp932')
            codec = None
        """)
        _testcapi.run_in_subinterp(code)

_codecs_jp.getcodec() is defined in Modules/cjkcodecs/cjkcodecs.h. Extract:

    cofunc = getmultibytecodec();
    ...
    codecobj = PyCapsule_New((void *)codec, PyMultibyteCodec_CAPSULE_NAME, NULL);
    if (codecobj == NULL)
        return NULL;

    r = PyObject_CallOneArg(cofunc, codecobj);
    Py_DECREF(codecobj);

getmultibytecodec() is the _multibytecodec.__create_codec() which is defined in Modules/cjkcodecs/multibytecodec.c. Simplified code:

    codec = PyCapsule_GetPointer(arg, PyMultibyteCodec_CAPSULE_NAME);
    _multibytecodec_state *state = _multibytecodec_get_state(module);
    self = PyObject_New(MultibyteCodecObject, state->multibytecodec_type);
    self->codec = codec;
    return (PyObject *)self;