bpo-38037: Fix reference counters in signal module

[ghost]

Only 3.8 and 3.9 branches are affected.

Use PyDict_SetItemString() in this PR.

https://bugs.python.org/issue38037

[vstinner]

LGTM.

@nanjekyejoannah: Would you mind to review it as well?

[nanjekyejoannah]

LGTM.

[vstinner]

I managed to reproduce https://bugs.python.org/issue38037#msg351196 crash using this change:

diff --git a/Objects/longobject.c b/Objects/longobject.c
index 4cf2b0726e..0ad2609882 100644
--- a/Objects/longobject.c
+++ b/Objects/longobject.c
@@ -16,10 +16,10 @@ class int "PyObject *" "&PyLong_Type"
 /*[clinic end generated code: output=da39a3ee5e6b4b0d input=ec0275e3422a36e3]*/
 
 #ifndef NSMALLPOSINTS
-#define NSMALLPOSINTS           257
+#define NSMALLPOSINTS           0
 #endif
 #ifndef NSMALLNEGINTS
-#define NSMALLNEGINTS           5
+#define NSMALLNEGINTS           0
 #endif
 
 _Py_IDENTIFIER(little);

Start Python, exit Python: Python does crash at exit.

I confirm that this change fix the crash.

[miss-islington]

Thanks @animalize for the PR, and @vstinner for merging it 🌮🎉.. I'm working now to backport this PR to: 3.8.
🐍🍒⛏🤖

[bedevere-bot]

GH-15779 is a backport of this pull request to the 3.8 branch.