Process import change from billiard since 3.4 multiprocessing Fix #2530

[https://github.com/celery/billiard/commit/c7eedbd0ee1498e76d4fa1affac5b
1a275660ee7]

celery/beat.py

@@ -21,7 +21,7 @@ from functools import total_ordering
 from threading import Event, Thread
 
 from billiard import ensure_multiprocessing
-from billiard.process import Process
+from billiard.context import Process
 from billiard.common import reset_signals
 from kombu.utils import cached_property, reprcall
 from kombu.utils.functional import maybe_evaluate