fix loop error.

Author: claudevdm

sdks/python/apache_beam/runners/interactive/user_pipeline_tracker.py

@@ -68,7 +68,7 @@ def evict(self, pipeline: beam.Pipeline) -> None:
   def clear(self) -> None:
     """Clears the tracker of all user and derived pipelines."""
     # Remove all local_tempdir of created pipelines.
-    for p in self._pid_to_pipelines.values():
+    for p in list(self._pid_to_pipelines.values()):
       shutil.rmtree(p.local_tempdir, ignore_errors=True)
 
     self._user_pipelines.clear()