[AIRFLOW-5514] No implicit optional flag for mypy

[TobKed]

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[TobKed]

PEP-484 says:

A past version of this PEP allowed type checkers to assume an optional type when the default value is None.
...
This is no longer the recommended behavior. Type checkers should move towards requiring the optional type to be made explicit.

Other references:
https://github.com/python/peps/pull/689/files
#5965 (comment)

[TobKed]

cc @mik-laj @potiuk @kaxil

[codecov-io]

Codecov Report

Merging #6141 into master will increase coverage by 0.03%.
The diff coverage is 97.46%.

@@            Coverage Diff             @@
##           master    #6141      +/-   ##
==========================================
+ Coverage   80.02%   80.05%   +0.03%     
==========================================
  Files         607      607              
  Lines       35032    35036       +4     
==========================================
+ Hits        28034    28049      +15     
+ Misses       6998     6987      -11

Impacted Files	Coverage Δ
airflow/operators/bigquery_to_mysql.py	74% <ø> (ø)	⬆️
airflow/api/common/experimental/trigger_dag.py	100% <ø> (ø)	⬆️
airflow/operators/slack_operator.py	100% <ø> (ø)	⬆️
airflow/kubernetes/pod_launcher.py	91.97% <ø> (ø)	⬆️
airflow/operators/hive_stats_operator.py	100% <ø> (ø)	⬆️
airflow/gcp/hooks/automl.py	99.17% <ø> (ø)	⬆️
airflow/gcp/operators/dataproc.py	81.68% <ø> (ø)	⬆️
airflow/gcp/hooks/translate.py	86.66% <ø> (ø)	⬆️
airflow/gcp/hooks/dataproc.py	55.25% <ø> (ø)	⬆️
airflow/operators/python_operator.py	95.75% <ø> (ø)	⬆️
... and 60 more

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[potiuk]

I love it!