Fix breaking change in Azure ServiceBus SDK

[c-w]

The Azure ServiceBus SDK upgrade from version 0.21.1 to 0.50.0 introduced a breaking change which affects the azureservicebus:// transport. This pull request makes kombu compatible with both old and new versions of the SDK. See the Azure ServiceBus SDK release notes for more background.

[c-w]

NB: the Travis build is failing due to lint errors in files that were not changed by this pull request: time.py, text.py, url.py, base.py, and test_timer.py. Not sure what's going on.

Perhaps someone could look into this, perhaps @georgepsarakis or @auvipy who helped with the Azure support pull requests in the past? Thanks in advance!

[auvipy]

would you mind fixing the flake8 error?

[auvipy]

@thedrow would you mind considering a RC release of kombu and celery 4.3 any time soon? if not you could document the release process with bumpversion so others can also contribute to the release process?

[codecov]

Codecov Report

Merging #998 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #998   +/-   ##
=======================================
  Coverage   88.59%   88.59%           
=======================================
  Files          63       63           
  Lines        6549     6549           
  Branches      781      781           
=======================================
  Hits         5802     5802           
  Misses        664      664           
  Partials       83       83

---

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[codecov]

Codecov Report

Merging #998 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #998   +/-   ##
=======================================
  Coverage   88.59%   88.59%           
=======================================
  Files          63       63           
  Lines        6549     6549           
  Branches      781      781           
=======================================
  Hits         5802     5802           
  Misses        664      664           
  Partials       83       83

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update b4f239c...e415158. Read the comment docs.