Deprecation warning due to invalid escape sequences

[tirkarthi]

Deprecation warnings are raised due to invalid escape sequences. This can be fixed by using raw strings or escaping the literals. pyupgrade also helps in automatic conversion : https://github.com/asottile/pyupgrade/

find . -iname '*.py' | grep -v example | xargs -P4 -I{} python3.8 -Wall -m py_compile {}
./celery_once/tasks.py:78: DeprecationWarning: invalid escape sequence \*
  """