Calling PyStructSequence_NewType() with a NULL field in the desc.doc parameter (explicitly allowed as per docs) leads to a crash in 

Objects/typeobject.c:2956
2956                size_t len = strlen(old_doc)+1;

where old_doc is NULL.
If the doc string is set, the call succeeds, but with a warning

(stdin):1: DeprecationWarning: builtin type G_SGROUP has no __module__ attribute

(where G_SGROUP is my new type), which did not happen in 3.6, and which I do not think can be suppressed by function call arguments.

It looks like the segfault was fixed in https://github.com/python/cpython/commit/88c2cfd9ffbcfc43fd1364f2984852a819547d43

as part of https://bugs.python.org/issue41832.

The code in this area of typeobject.c looks a bit different, now, but the backport seems simple?

Simple testcase:

#include <stdio.h>
#include <Python.h>
int main() {
    Py_Initialize();
    PyStructSequence_Field fields[2] = {
        {NULL, NULL}
    };
    PyStructSequence_Desc d = {"test", NULL, &fields[0], 0};
    PyStructSequence_NewType(&d);
    Py_Finalize();
}

Segfault reproducible on 3.8 and 3.9

This missed the train for inclusion in 3.8. There's still time for a backport for 3.9.

Changing PyType_FromSpec* to accept NULL has an issue: extensions built and tested with 3.9.5 would not work with the earlier 3.9s.

I'll send a PR to fix just PyStructSequence_NewType.

New changeset ec18362f6a7fdc02f9f982872fc1006bca31627d by Petr Viktorin in branch '3.9':
[3.9] bpo-42083: Allow NULL doc in PyStructSequence_NewType (#25896)
https://github.com/python/cpython/commit/ec18362f6a7fdc02f9f982872fc1006bca31627d

Steve, thank you for your invaluable investigation. Thanks Petr for a better fix - your issue didn't come to my mind at the time.

Since all PRs have landed and the fix should arrive in Python 3.9.6, I am closing this issue. Please don't hesitate to reopen this if anyone feels it needs revisiting. Thanks!