Requirements: upgrade Celery

[humitos]

I added this note in #10503

# NOTE: we have to pin to this version because Celery>=5.3.x detects our ``ConfigError`` exception is not picklable
# and creates a ``UnpickleableExceptionWrapper`` which is not what our code expects.
# This may be due to a Python bug which can't pickle/unpickle the exception properly.
# We could consider either:
#  - handle ``UnpickleableExceptionWrapper`` in our code and inspect what's the inner exception
#  - find a way to disable pickling the exceptions
#  - make our custom exception picklable
#
# References:
#  - https://github.com/celery/celery/pull/8149
#  - https://github.com/celery/celery/blob/2b4b500ca1212016824a5fa2996cfb752f0763a7/celery/utils/serialization.py#L154
#  - https://github.com/python/cpython/issues/76877
#
# Reproducible example:
# >>> import pickle
# >>> class MyException(Exception):
# ...   def __init__(self, a, b):
# ...     self.a = a
# ...     self.b = b
# ...     super().__init__(a)
# ...
# >>> pickle.loads(pickle.dumps(MyException("a", "b")))
# Traceback (most recent call last):
#   File "<stdin>", line 1, in <module>
# TypeError: MyException.__init__() missing 1 required positional argument: 'b'
# >>>
celery==5.2.7

We need to do some work here to be able to upgrade Celery to 5.3.x