[hotfix] Fix missing UserAgent (#5)

* Update __init__.py

* format

src/agentscope/agents/__init__.py

@@ -1,10 +1,11 @@
 # -*- coding: utf-8 -*-
-""" Import all agent related modules in the package. """
+""" Import all agent-related modules in the package. """
 from typing import Callable
 from .agent import AgentBase
 from .rpc_agent import RpcAgentBase
 from .dialog_agent import DialogAgent
 from .dict_dialog_agent import DictDialogAgent
+from .user_agent import UserAgent
 
 # todo: convert Operator to a common base class for AgentBase and PipelineBase
 _Operator = Callable[..., dict]
@@ -15,4 +16,5 @@
     "RpcAgentBase",
     "DialogAgent",
     "DictDialogAgent",
+    "UserAgent",
 ]