[AIRFLOW-1797] S3Hook.load_string didn't work on Python3

[ashb]

Dear Airflow maintainers,

Please accept this PR. I understand that it will not be reviewed until I have checked off all the steps below!

JIRA

• My PR addresses the following Airflow JIRA issues and references them in the PR title. https://issues.apache.org/jira/browse/AIRFLOW-1797

Description

• With the switch to Boto3 ([AIRFLOW-1520] Boto3 S3Hook, S3Log #2532) we now need the content to be bytes, not a string. On Python2 there is no difference, but for Python3 this matters.

Tests

• My PR adds the following unit tests: New tests/hooks/test_s3_hook.py file covering load_string and read_key. Moved one existing test out of tests/core.py

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
  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"

[codecov-io]

Codecov Report

Merging #2771 into master will increase coverage by 0.05%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #2771      +/-   ##
==========================================
+ Coverage   73.26%   73.31%   +0.05%     
==========================================
  Files         157      157              
  Lines       11934    11934              
==========================================
+ Hits         8744     8750       +6     
+ Misses       3190     3184       -6

Impacted Files	Coverage Δ
airflow/hooks/S3_hook.py	51.19% <100%> (+20.23%)	⬆️
airflow/task_runner/bash_task_runner.py	93.33% <0%> (-6.67%)	⬇️
airflow/utils/helpers.py	53.44% <0%> (-2.88%)	⬇️
airflow/jobs.py	79.23% <0%> (-0.45%)	⬇️

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