put w/ space around == operator as advised by flake8 test

Reference: #3321 (comment)

luigi/db_task_history.py

@@ -192,7 +192,7 @@ def find_task_by_task_id(self, task_id, session=None):
         Find task with the given task ID.
         """
         with self._session(session) as session:
-            return session.query(TaskRecord).filter(TaskRecord.task_id==task_id).all()[-1]
+            return session.query(TaskRecord).filter(TaskRecord.task_id == task_id).all()[-1]
 
 
 class TaskParameter(Base):  # type: ignore