Don't serialize I/O wrappers

This removes warnings on `config.get`, where the `logging` configuration
key may also contain the current logging stream and we end up with a
JSONDecodeError when trying to serialize it.

platypush/message/__init__.py

@@ -1,6 +1,7 @@
 from abc import ABC, abstractmethod
 import decimal
 import datetime
+import io
 import logging
 import inspect
 import json
@@ -66,6 +67,10 @@ class Message:
             if isinstance(obj, JSONAble):
                 return obj.to_json()
 
+            # Don't serialize I/O wrappers/objects
+            if isinstance(obj, io.IOBase):
+                return None
+
             try:
                 return super().default(obj)
             except Exception as e: