[AIRFLOW-4200][AIP-3-subtask-4] Remove all __future__ imports

[zhongjiajie]

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-4200
  • 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

Code Quality

• Passes flake8

[Fokko]

Can you rebase onto master?

[zhongjiajie]

CI failed because our CI process have py2 envirement, I submit a PR #5090 to remove them. @Fokko

[Fokko]

It looks like the K8s tests still use Python 2.7:

+ airflow initdb
Traceback (most recent call last):
  File "/usr/local/bin/airflow", line 21, in <module>
    from airflow import configuration
  File "/usr/local/lib/python2.7/dist-packages/airflow/__init__.py", line 36, in <module>
    from airflow import settings, configuration as conf
  File "/usr/local/lib/python2.7/dist-packages/airflow/settings.py", line 32, in <module>
    from airflow.utils.sqlalchemy import setup_event_handlers
  File "/usr/local/lib/python2.7/dist-packages/airflow/utils/sqlalchemy.py", line 22, in <module>
    import json
  File "/usr/local/lib/python2.7/dist-packages/airflow/utils/json.py", line 27, in <module>
    class AirflowJsonEncoder(json.JSONEncoder):
AttributeError: 'module' object has no attribute 'JSONEncoder'

[codecov-io]

Codecov Report

Merging #5020 into master will increase coverage by 0.22%.
The diff coverage is n/a.

@@            Coverage Diff            @@
##           master   #5020      +/-   ##
=========================================
+ Coverage   78.08%   78.3%   +0.22%     
=========================================
  Files         465     465              
  Lines       29803   29706      -97     
=========================================
- Hits        23271   23262       -9     
+ Misses       6532    6444      -88

Impacted Files	Coverage Δ
airflow/configuration.py	90.62% <ø> (-0.13%)	⬇️
airflow/utils/log/logging_mixin.py	96.1% <ø> (-0.2%)	⬇️
airflow/bin/cli.py	66.63% <ø> (-0.04%)	⬇️
...example_dags/example_kubernetes_executor_config.py	0% <ø> (ø)	⬆️
airflow/utils/state.py	96.42% <ø> (-0.13%)	⬇️
airflow/contrib/operators/cassandra_to_gcs.py	63.09% <ø> (-0.22%)	⬇️
airflow/dag/base_dag.py	60% <ø> (-6.67%)	⬇️
airflow/utils/helpers.py	82.39% <ø> (-0.49%)	⬇️
airflow/settings.py	83.87% <ø> (-0.51%)	⬇️
airflow/hooks/hive_hooks.py	75.06% <ø> (+1.78%)	⬆️
... and 39 more

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[zhongjiajie]

CI green, PTAL @Fokko @potiuk @XD-DENG thanks.

[zhongjiajie]

Thanks @potiuk