Annotate known magic-method return types (#9131)

stubs/httplib2/httplib2/__init__.pyi

@@ -30,7 +30,7 @@ class Authentication:
     def __gt__(self, auth): ...
     def __le__(self, auth): ...
     def __ge__(self, auth): ...
-    def __bool__(self): ...
+    def __bool__(self) -> bool: ...
 
 class BasicAuthentication(Authentication):
     def __init__(self, credentials, host, request_uri, headers, response, content, http) -> None: ...