FYI, I think this is the culprit.
In Python/pylifecycle.c:

static char*
get_codec_name(const char *encoding)
{
    char *name_utf8, *name_str;
    PyObject *codec, *name = NULL;

    codec = _PyCodec_Lookup(encoding);
    if (!codec)
        goto error;

    name = _PyObject_GetAttrId(codec, &PyId_name);
    Py_CLEAR(codec);
    if (!name)
        goto error;

    name_utf8 = _PyUnicode_AsString(name);
    if (name_utf8 == NULL)
        goto error;
    name_str = _PyMem_RawStrdup(name_utf8);
    Py_DECREF(name);
    if (name_str == NULL) {
        PyErr_NoMemory();
        return NULL;
    }
    return name_str;

error:
    Py_XDECREF(codec);
    Py_XDECREF(name);
    return NULL;
}

If I figure out a working patch I'll post it here