[AIRFLOW-4947] Remove six types

[mik-laj]

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-XXX
  • 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).
  • In case you are adding a dependency, check if the license complies with the ASF 3rd Party License Policy.

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 (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.
  • All the public functions and the classes in the PR contain docstrings that explain what it does
  • If you implement backwards incompatible changes, please leave a note in the Updating.md so we can assign it to a appropriate release

Code Quality

• Passes flake8

[potiuk]

LGTM

[potiuk]

You have to fix the imports though (flake)

[codecov-io]

Codecov Report

Merging #5581 into master will decrease coverage by 0.01%.
The diff coverage is 93.93%.

@@            Coverage Diff             @@
##           master    #5581      +/-   ##
==========================================
- Coverage   79.06%   79.05%   -0.02%     
==========================================
  Files         489      489              
  Lines       30711    30684      -27     
==========================================
- Hits        24282    24257      -25     
+ Misses       6429     6427       -2

Impacted Files	Coverage Δ
airflow/contrib/sensors/gcp_transfer_sensor.py	100% <ø> (ø)	⬆️
airflow/contrib/hooks/bigquery_hook.py	61.38% <ø> (-0.07%)	⬇️
airflow/contrib/hooks/gcp_transfer_hook.py	90.41% <ø> (-0.07%)	⬇️
airflow/contrib/hooks/qubole_hook.py	55% <0%> (-0.32%)	⬇️
airflow/stats.py	69.23% <0%> (-0.47%)	⬇️
airflow/models/baseoperator.py	94.7% <100%> (-0.02%)	⬇️
airflow/bin/cli.py	66.42% <100%> (-0.04%)	⬇️
airflow/models/dag.py	91.83% <100%> (-0.02%)	⬇️
...flow/contrib/operators/gcp_cloud_build_operator.py	100% <100%> (ø)	⬆️
.../contrib/operators/jenkins_job_trigger_operator.py	67.67% <100%> (-0.33%)	⬇️
... and 18 more

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