There was such bug in CPython stdlib, in the _testcapi module, see commit 310e2d25170a88ef03f6fd31efcc899fe062da2c of bpo-36854:

commit 310e2d25170a88ef03f6fd31efcc899fe062da2c
Author: Victor Stinner <vstinner@python.org>
Date:   Fri Nov 22 10:58:00 2019 +0100

    bpo-36854: Fix refleak in subinterpreter (GH-17331)
    
    finalize_interp_clear() now explicitly clears the codec registry and
    then trigger a GC collection to clear all references.

diff --git a/Modules/_testcapimodule.c b/Modules/_testcapimodule.c
index baa6907b7e..0908f3457f 100644
--- a/Modules/_testcapimodule.c
+++ b/Modules/_testcapimodule.c
@@ -6721,11 +6721,14 @@ PyInit__testcapi(void)
     PyModule_AddObject(m, "instancemethod", (PyObject *)&PyInstanceMethod_Type);
 
     PyModule_AddIntConstant(m, "the_number_three", 3);
+    PyObject *v;
 #ifdef WITH_PYMALLOC
-    PyModule_AddObject(m, "WITH_PYMALLOC", Py_True);
+    v = Py_True;
 #else
-    PyModule_AddObject(m, "WITH_PYMALLOC", Py_False);
+    v = Py_False;
 #endif
+    Py_INCREF(v);
+    PyModule_AddObject(m, "WITH_PYMALLOC", v);
 
     TestError = PyErr_NewException("_testcapi.error", NULL, NULL);
     Py_INCREF(TestError);