mark fb303.fb303.FacebookService.Client as a context manager (#5542)

Author: arthaud

stubs/fb303/fb303/FacebookService.pyi

@@ -1,4 +1,4 @@
-from typing import Any
+from typing import Any, ContextManager
 
 from thrift.Thrift import TProcessor  # type: ignore
 
@@ -19,7 +19,7 @@ class Iface:
     def reinitialize(self): ...
     def shutdown(self): ...
 
-class Client(Iface):
+class Client(Iface, ContextManager[Client]):
     def __init__(self, iprot, oprot=...) -> None: ...
     def getName(self): ...
     def send_getName(self): ...