[AIRFLOW-2384] Set flask to 0.12.2

[ese]

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JIRA

• https://issues.apache.org/jira/browse/AIRFLOW-2384

Description

• Here are some details about my PR, including screenshots of any UI changes:

Tests

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

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"

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.

Code Quality

• Passes git diff upstream/master -u -- "*.py" | flake8 --diff

[codecov-io]

Codecov Report

Merging #3272 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #3272   +/-   ##
=======================================
  Coverage   75.78%   75.78%           
=======================================
  Files         197      197           
  Lines       14699    14699           
=======================================
  Hits        11140    11140           
  Misses       3559     3559

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[Fokko]

Oops, I also fixed this is #3277