[AIRFLOW-5599] Improve Python 3 support in MLEngine integration

[mik-laj]

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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-5599
  • In case you are fixing a typo in the documentation you can prepend your commit with [AIRFLOW-XXX], code changes always need a Jira issue.
  • In case you are proposing a fundamental code change, you need to create an Airflow Improvement Proposal (AIP).
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Description

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Tests

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

Commits

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Documentation

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[codecov-io]

Codecov Report

Merging #6262 into master will not change coverage.
The diff coverage is 50%.

@@           Coverage Diff           @@
##           master    #6262   +/-   ##
=======================================
  Coverage   80.05%   80.05%           
=======================================
  Files         610      610           
  Lines       35264    35264           
=======================================
  Hits        28232    28232           
  Misses       7032     7032

Impacted Files	Coverage Δ
airflow/gcp/utils/mlengine_prediction_summary.py	0% <0%> (ø)	⬆️
airflow/gcp/utils/mlengine_operator_utils.py	95.34% <100%> (ø)	⬆️

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