Adding these two lines to /Objects/longobject.c will disable the "preallocated small integer pool":

    #define NSMALLPOSINTS  0
    #define NSMALLNEGINTS  0

Then run this reproduce code (attached):

    from enum import IntEnum
    import _signal

    class Handlers(IntEnum):
        A = _signal.SIG_DFL
        B = _signal.SIG_IGN

When the interpreter exits, will get this error:

    d:\dev\cpython\PCbuild\win32>python_d.exe d:\a.py
    d:\dev\cpython\include\object.h:541: _Py_NegativeRefcount: Assertion failed: object has negative ref count
    <object: freed>
    Fatal Python error: _PyObject_AssertFailed

    Current thread 0x0000200c (most recent call first):

3.8 and 3.9 branches are affected.
I'm sorry, this issue is beyond my ability.

I did a Git bisect, this is the first bad commit:
https://github.com/python/cpython/commit/9541bd321a94f13dc41163a5d7a1a847816fac84

nosy involved mates.

This issue is a Python 3.8 regression.

Joannah: Would you mind to have a look?

    x = DefaultHandler = PyLong_FromVoidPtr((void *)SIG_DFL);
    if (PyModule_AddObject(m, "SIG_DFL", x))
        goto finally;

This change is not easy to read.

DefaultHandler must be a strong reference: finisignal() calls Py_CLEAR(DefaultHandler);

Previously, the code uses PyDict_SetItemString(d, "SIG_DFL", x): PyDict_SetItemString increases the reference counter, whereas PyModule_AddObject leaves the reference counter unchanged (yeah, it's a strange/bad C API).

I guess than an INCREF() is needed somewhere.

Compare it to:

int
PyModule_AddIntConstant(PyObject *m, const char *name, long value)
{
    PyObject *o = PyLong_FromLong(value);
    if (!o)
        return -1;
    if (PyModule_AddObject(m, name, o) == 0)
        return 0;
    Py_DECREF(o);
    return -1;
}

I will look after my next meeting which is in 10 minutes. In the meantime have you perused this PR : https://github.com/python/cpython/pull/15701 If it can solve this?

New changeset 77643c486fd22d8030e0d82c13012684b4ab6df1 by Victor Stinner (animalize) in branch 'master':
bpo-38037: Fix reference counters in signal module (GH-15753)
https://github.com/python/cpython/commit/77643c486fd22d8030e0d82c13012684b4ab6df1

New changeset b150d0bf1bb4c3203bb3293625e32aed01b25887 by Miss Islington (bot) in branch '3.8':
bpo-38037: Fix reference counters in signal module (GH-15753)
https://github.com/python/cpython/commit/b150d0bf1bb4c3203bb3293625e32aed01b25887

Thanks for the fix animalize.