Added "stack_info" as reserved attribute (introduced in Python 3.2)

Author: svisser

src/pythonjsonlogger/jsonlogger.py

@@ -19,7 +19,7 @@
     'args', 'asctime', 'created', 'exc_info', 'exc_text', 'filename',
     'funcName', 'levelname', 'levelno', 'lineno', 'module',
     'msecs', 'message', 'msg', 'name', 'pathname', 'process',
-    'processName', 'relativeCreated', 'thread', 'threadName')
+    'processName', 'relativeCreated', 'stack_info', 'thread', 'threadName')
 
 RESERVED_ATTR_HASH = dict(zip(RESERVED_ATTRS, RESERVED_ATTRS))