Fix linter errors

esrally/driver/driver.py

@@ -834,6 +834,7 @@ def joinpoint_reached(self, worker_id, worker_local_timestamp, task_allocations)
                 self.telemetry.on_benchmark_stop()
                 self.logger.info("All steps completed.")
                 # Some metrics store implementations return None because no external representation is required.
+                # pylint: disable=assignment-from-none
                 m = self.metrics_store.to_externalizable(clear=True)
                 self.logger.debug("Closing metrics store...")
                 self.metrics_store.close()
@@ -866,6 +867,7 @@ def move_to_next_task(self, workers_curr_step):
             #             (it doesn't matter too much if we're a few ms off).
             waiting_period = 1.0
         # Some metrics store implementations return None because no external representation is required.
+        # pylint: disable=assignment-from-none
         m = self.metrics_store.to_externalizable(clear=True)
         self.driver_actor.on_task_finished(m, waiting_period)
         # Using a perf_counter here is fine also in the distributed case as we subtract it from `master_received_msg_at` making it