[AIRFLOW-4819] Fix singleton-comparision errors in pylint

[mik-laj]

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[codecov-io]

Codecov Report

Merging #5440 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master    #5440   +/-   ##
=======================================
  Coverage   79.11%   79.11%           
=======================================
  Files         488      488           
  Lines       30582    30582           
=======================================
  Hits        24194    24194           
  Misses       6388     6388

Impacted Files	Coverage Δ
airflow/jobs/scheduler_job.py	70.32% <ø> (ø)	⬆️
airflow/www/security.py	92.3% <ø> (ø)	⬆️
airflow/models/dagrun.py	96.48% <ø> (ø)	⬆️
airflow/models/dag.py	91.8% <100%> (ø)	⬆️

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[BasPH]

@mik-laj FYI SQLAlchemy has some linter-friendly functions to avoid e.g. == None and make linters happy:

query.filter(User.name == None)

# linter friendly equivalent:
query.filter(User.name.is_(None))

More here: https://docs.sqlalchemy.org/en/13/orm/tutorial.html#common-filter-operators