[AIRFLOW-1402] Cleanup SafeConfigParser DeprecationWarning

[ronfung]

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]https://issues.apache.org/jira/browse/AIRFLOW-1402) issues and references them in the PR title.

Description

• Here are some details about my PR, including screenshots of any UI changes:
• use ConfigParser for python 3.x
  DeprecationWarning: The SafeConfigParser class has been renamed to ConfigParser in Python 3.2. This alias will be removed in future versions. Use ConfigParser directly instead.

Tests

• Test done with ./run_unit_tests.sh on python 2.x and python 3.x pyenv

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 #2435 into master will decrease coverage by 0.03%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #2435      +/-   ##
==========================================
- Coverage   69.31%   69.28%   -0.04%     
==========================================
  Files         146      146              
  Lines       11242    11244       +2     
==========================================
- Hits         7792     7790       -2     
- Misses       3450     3454       +4

Impacted Files	Coverage Δ
airflow/configuration.py	83.96% <100%> (+0.15%)	⬆️
airflow/jobs.py	75.05% <0%> (-0.41%)	⬇️

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[criccomini]

LGTM