fix style issue found by flake8

src/nominatim_db/db/query_pool.py

@@ -18,6 +18,7 @@ LOG = logging.getLogger()
 
 QueueItem = Optional[Tuple[psycopg.abc.Query, Any]]
 
+
 class QueryPool:
     """ Pool to run SQL queries in parallel asynchronous execution.
 
@@ -32,7 +33,6 @@ class QueryPool:
         self.pool = [asyncio.create_task(self._worker_loop(dsn, **conn_args))
                      for _ in range(pool_size)]
 
-
     async def put_query(self, query: psycopg.abc.Query, params: Any) -> None:
         """ Schedule a query for execution.
         """
@@ -41,7 +41,6 @@ class QueryPool:
         self.wait_time += time.time() - tstart
         await asyncio.sleep(0)
 
-
     async def finish(self) -> None:
         """ Wait for all queries to finish and close the pool.
         """
@@ -57,7 +56,6 @@ class QueryPool:
             if excp is not None:
                 raise excp
 
-
     async def _worker_loop(self, dsn: str, **conn_args: Any) -> None:
         conn_args['autocommit'] = True
         aconn = await psycopg.AsyncConnection.connect(dsn, **conn_args)
@@ -78,10 +76,8 @@ class QueryPool:
                                  str(item[0]), str(item[1]))
                         # item is still valid here, causing a retry
 
-
     async def __aenter__(self) -> 'QueryPool':
         return self
 
-
     async def __aexit__(self, exc_type: Any, exc_value: Any, traceback: Any) -> None:
         await self.finish()