yangck/celery @ v4.2.0

Omer Katz 120770929f Install isort from git. Fix import order.		7 lat temu
..
__init__.py	d59518f5fb Make __all__ immutable. (#4315)	7 lat temu
certificate.py	120770929f Install isort from git. Fix import order.	7 lat temu
key.py	e6a6cced7f isort.	7 lat temu
serialization.py	120770929f Install isort from git. Fix import order.	7 lat temu
utils.py	e6a6cced7f isort.	7 lat temu