[AIRFLOW-5883] Don't use .count() from sqlalchemy to count

[saguziel]

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[KevinYang21]

Love this change.

[codecov-io]

Codecov Report

Merging #6532 into master will decrease coverage by 0.29%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master    #6532     +/-   ##
=========================================
- Coverage   84.08%   83.78%   -0.3%     
=========================================
  Files         635      635             
  Lines       36849    36850      +1     
=========================================
- Hits        30983    30875    -108     
- Misses       5866     5975    +109

Impacted Files	Coverage Δ
airflow/sensors/external_task_sensor.py	85.96% <100%> (+0.25%)	⬆️
airflow/models/dag.py	90.87% <100%> (ø)	⬆️
airflow/models/taskinstance.py	93.79% <100%> (ø)	⬆️
airflow/kubernetes/volume_mount.py	44.44% <0%> (-55.56%)	⬇️
airflow/kubernetes/volume.py	52.94% <0%> (-47.06%)	⬇️
airflow/kubernetes/pod_launcher.py	45.25% <0%> (-46.72%)	⬇️
airflow/kubernetes/kube_client.py	33.33% <0%> (-41.67%)	⬇️
...rflow/contrib/operators/kubernetes_pod_operator.py	70.14% <0%> (-28.36%)	⬇️
airflow/utils/dag_processing.py	58.48% <0%> (+0.32%)	⬆️

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[feluelle]

This is great. Thanks @saguziel