Research/test pinning kombu version

[lbeaufort]

Celery 4.1.1 depends on kombu >=4.2.0,<5.0. Kombu's breaking changes in minor upgrades have bitten us hard, twice. (See #3607 and f83ae22)

Kombu doesn't seem to follow semantic versioning. From another celery user:

“It would seem that you want people to pin specific versions of kombu when using celery. That’s something that I will be doing in the future as kombu changes have included breaking changes outside of major version releases.”

We should research/test pinning kombu versions in our requirements file to see if that will force celery to use that version.

Completion criteria

• Try pinning kombu==4.3.0 in requirements.txt
• Do a fresh pip install -r requirements.txt
• Do a pip freeze to see what version of kombu in in your environment - celery 4.1.1 would normally grab kombu 4.4.0 but we want to ensure that pinning at 4.3.0 actually works.
• If this works, add a comment to the celery line in requirements.txt that if we update celery we should check which version(s) of kombu will work, and pin the right version.

[jason-upchurch]

• Do a fresh pip install -r requirements.txt

Installing collected packages: kombu
  Found existing installation: kombu 4.5.0
    Uninstalling kombu-4.5.0:
      Successfully uninstalled kombu-4.5.0
Successfully installed kombu-4.3.0

edited to show output of pip install -r requirements.txt step

[jason-upchurch]

updated requirements.txt with comment on celery to check kombu version compatibility.

[pkfec]

@jason-upchurch Could you submit a pull request and have someone review your changes?