bpo-40077: Fix potential refleaks of _json: traverse memo (GH-19344)

Fix possible refleaks in _json module, memo of PyScannerObject
should be traversed.

Author: shihai1991

Misc/NEWS.d/next/Core and Builtins/2020-04-04-12-43-19.bpo-40077.m15TTX.rst

@@ -0,0 +1 @@
+Fix possible refleaks in :mod:`_json`, memo of PyScannerObject should be traversed.

Modules/_json.c

@@ -652,6 +652,7 @@ scanner_traverse(PyScannerObject *self, visitproc visit, void *arg)
     Py_VISIT(self->parse_float);
     Py_VISIT(self->parse_int);
     Py_VISIT(self->parse_constant);
+    Py_VISIT(self->memo);
     return 0;
 }