Added celerybeat.pid (#3229)

Python.gitignore

@@ -91,8 +91,9 @@ ipython_config.py
 #   install all needed dependencies.
 #Pipfile.lock
 
-# celery beat schedule file
+# Celery stuff
 celerybeat-schedule
+celerybeat.pid
 
 # SageMath parsed files
 *.sage.py