When using the `test-with-buildbots` label in GH-19149 (which involved no C changes), a failure occurred in test_asyncio for several of the refleak buildbots. Here's the output of a few:

AMD64 Fedora Stable Refleaks PR:

test_asyncio leaked [3, 3, 27] references, sum=33
test_asyncio leaked [3, 3, 28] memory blocks, sum=34
2 tests failed again:
    test__xxsubinterpreters test_asyncio
== Tests result: FAILURE then FAILURE ==

AMD64 RHEL8 Refleaks PR:

test_asyncio leaked [3, 3, 3] references, sum=9
test_asyncio leaked [3, 3, 3] memory blocks, sum=9
2 tests failed again:
    test__xxsubinterpreters test_asyncio
== Tests result: FAILURE then FAILURE ==

RHEL7 Refleaks PR:

test_asyncio leaked [3, 3, 3] references, sum=9
test_asyncio leaked [3, 3, 3] memory blocks, sum=9
2 tests failed again:
    test__xxsubinterpreters test_asyncio
== Tests result: FAILURE then FAILURE ==


I'm unable to replicate it locally, but I think I may have located a subtle, uncommon refleak in `future_add_done_callback()`, within _asynciomodule.c. Specifically:

```
            PyObject *tup = PyTuple_New(2);
            if (tup == NULL) {
                return NULL;
            }
            Py_INCREF(arg);
            PyTuple_SET_ITEM(tup, 0, arg);
            Py_INCREF(ctx);
            PyTuple_SET_ITEM(tup, 1, (PyObject *)ctx);

            if (fut->fut_callbacks != NULL) {
                int err = PyList_Append(fut->fut_callbacks, tup);
                if (err) {
                    Py_DECREF(tup);
                    return NULL;
                }
                Py_DECREF(tup);
            }
            else {
                fut->fut_callbacks = PyList_New(1);
                if (fut->fut_callbacks == NULL) {
                    // Missing ``Py_DECREF(tup);`` ?
                    return NULL;
                }
```

(The above code is located at: https://github.com/python/cpython/blob/7668a8bc93c2bd573716d1bea0f52ea520502b28/Modules/_asynciomodule.c#L664-L685)

In the above conditional for "if (fut->fut_callbacks == NULL)", it appears that `tup` is pointing to a non-NULL new reference at this point, and thus should be decref'd prior to returning NULL. Otherwise, it seems like it could be leaked.

But, I would appreciate it if someone could double check this (the C-API isn't an area I'm experienced); particularly since this code has been in place for a decent while (since 3.7). I _suspect_ it's gone undetected and only failed intermittently because this specific ``return NULL`` path is rather uncommon.

I'd be glad to open a PR to address the issue, assuming I'm not missing something with the above refleak. Otherwise, feel free to correct me.