[AIRFLOW-1967] Update Celery to 4.0.2

[Fokko]

Hi all,

I'd like to update Celery to version 4.0.2. While updating my Docker container to version 1.9, I caught this error:

worker_1     | [2018-01-03 10:34:29,934: CRITICAL/MainProcess] Unrecoverable error: TypeError("'<=' not supported between instances of 'NoneType' and 'int'",)
worker_1     | Traceback (most recent call last):
worker_1     |   File "/usr/local/lib/python3.6/site-packages/celery/worker/worker.py", line 203, in start
worker_1     |     self.blueprint.start(self)
worker_1     |   File "/usr/local/lib/python3.6/site-packages/celery/bootsteps.py", line 115, in start
worker_1     |     self.on_start()
worker_1     |   File "/usr/local/lib/python3.6/site-packages/celery/apps/worker.py", line 143, in on_start
worker_1     |     self.emit_banner()
worker_1     |   File "/usr/local/lib/python3.6/site-packages/celery/apps/worker.py", line 159, in emit_banner
worker_1     |     string(self.colored.reset(self.extra_info() or '')),
worker_1     |   File "/usr/local/lib/python3.6/site-packages/celery/apps/worker.py", line 188, in extra_info
worker_1     |     if self.loglevel <= logging.INFO:
worker_1     | TypeError: '<=' not supported between instances of 'NoneType' and 'int'

This is because I've been running Python 2 in my local environments, and the Docker image is Python 3:
https://github.com/puckel/docker-airflow/pull/143/files

This is the issue in Celery:
https://github.com/celery/celery/blob/0dde9df9d8dd5dbbb97ef75a81757bc2d9a4b33e/Changelog#L145

Make sure you have checked all steps below.

JIRA

• My PR addresses the following Airflow JIRA issues and references them in the PR title. For example, "[AIRFLOW-XXX] My Airflow PR"
  • https://issues.apache.org/jira/browse/AIRFLOW-XXX

Description

• Here are some details about my PR, including screenshots of any UI changes:

Tests

• My PR adds the following unit tests OR does not need testing for this extremely good reason:

Commits

• My commits all reference JIRA issues in their subject lines, and I have squashed multiple commits if they address the same issue. In addition, my commits follow the guidelines from "How to write a good git commit message":

  1. Subject is separated from body by a blank line
  2. Subject is limited to 50 characters
  3. Subject does not end with a period
  4. Subject uses the imperative mood ("add", not "adding")
  5. Body wraps at 72 characters
  6. Body explains "what" and "why", not "how"

• Passes git diff upstream/master -u -- "*.py" | flake8 --diff

[r39132]

LGTM!