[AIRFLOW-3419] S3_hook.select_key is broken on Python3

[ashb]

Fix actual return type of select_key function

Re-opening #4571 after I messed up trying to rebase OP's branch. I have preserved the original author and commit time.

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-3419

Description

• Here are some details about my PR, including screenshots of any UI changes:
  Fixes S3_hook.select_key return type to str instead of bytes

Tests

• My PR adds the following unit tests OR does not need testing for this extremely good reason:
  original test changed to match actual types

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"

Documentation

• In case of new functionality, my PR adds documentation that describes how to use it.
  • When adding new operators/hooks/sensors, the autoclass documentation generation needs to be added.
  • All the public functions and the classes in the PR contain docstrings that explain what it does

Code Quality

• Passes flake8

[codecov-io]

Codecov Report

Merging #4970 into master will increase coverage by 0.64%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #4970      +/-   ##
==========================================
+ Coverage   75.74%   76.38%   +0.64%     
==========================================
  Files         458      458              
  Lines       29856    31314    +1458     
==========================================
+ Hits        22613    23918    +1305     
- Misses       7243     7396     +153

Impacted Files	Coverage Δ
airflow/hooks/S3_hook.py	93.03% <100%> (ø)	⬆️
...rflow/contrib/operators/kubernetes_pod_operator.py	92.77% <0%> (-5.85%)	⬇️
airflow/contrib/operators/aws_athena_operator.py	67.69% <0%> (-2.77%)	⬇️
airflow/hooks/hdfs_hook.py	91.8% <0%> (-0.7%)	⬇️
...etes_request_factory/kubernetes_request_factory.py	72.58% <0%> (-0.06%)	⬇️
airflow/contrib/kubernetes/pod.py	100% <0%> (ø)	⬆️
airflow/contrib/operators/gcs_to_bq.py	0% <0%> (ø)	⬆️
airflow/models/connection.py	66.48% <0%> (+0.57%)	⬆️
airflow/bin/cli.py	68.21% <0%> (+1.2%)	⬆️
airflow/contrib/kubernetes/secret.py	26.31% <0%> (+1.31%)	⬆️
... and 7 more

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