[AIRFLOW-4895] Import Iterable from collections.abc to fix DeprecationWarning in Python 3.7

[tirkarthi]

Jira

• My PR addresses the following Airflow Jira issues and references them in the PR title.

Description

Import Iterable from collections.abc to fix DeprecationWarning in Python 3.7. Use collections to ensure compatibility for Python 3.3 and below.

Tests

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

This fixes a DeprecationWarning and doesn't make code change

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 (not including Jira issue reference)
  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"

Code Quality

• Passes flake8

[ashb]

The mimimum Python version we support is now 3.5 so I don't think we need this. (unless we also want to backport this to 1.10.x, where we still support py2.7)

[tirkarthi]

Thanks, it's also present in 1.10.x branch. The fix was based on https://issues.apache.org/jira/browse/AIRFLOW-3009 where similar problem was fixed in master and also seemed to be backported to 1.10.3.

[codecov-io]

Codecov Report

Merging #5533 into master will decrease coverage by <.01%.
The diff coverage is 50%.

@@            Coverage Diff             @@
##           master    #5533      +/-   ##
==========================================
- Coverage   79.07%   79.07%   -0.01%     
==========================================
  Files         489      489              
  Lines       30744    30747       +3     
==========================================
+ Hits        24312    24313       +1     
- Misses       6432     6434       +2

Impacted Files	Coverage Δ
airflow/utils/helpers.py	82.58% <50%> (-0.98%)	⬇️

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