Convert to __call__() as coroutine interface

Required PR: pulp/pulp#3559

https://pulp.plan.io/issues/3890
re #3890

pulp_file/app/tasks/synchronizing.py

@@ -49,7 +49,7 @@ def __init__(self, remote):
         """
         self.remote = remote
 
-    async def gen_declarative_content(self, in_q, out_q):
+    async def __call__(self, in_q, out_q):
         """
         Build and emit `DeclarativeContent` from the Manifest data.