gh-120244: Fix re.sub() reference leak (GH-120245)

Author: neonene

Misc/NEWS.d/next/Library/2024-06-08-09-45-31.gh-issue-120244.8o9Dzr.rst

@@ -0,0 +1 @@
+Fix memory leak in :func:`re.sub()` when the replacement string contains backreferences.

Modules/_sre/sre.c

@@ -1622,6 +1622,7 @@ _sre_template_impl(PyObject *module, PyObject *pattern, PyObject *template)
         }
         self->items[i].literal = Py_XNewRef(literal);
     }
+    PyObject_GC_Track(self);
     return (PyObject*) self;
 
 bad_template: