[AIRFLOW-6080] fix type error from new version of mypy

[houqp]

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Description

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This PR fixes all errors reported by new version of mypy and updates CI to run mypy 0.740.

Tests

• My PR adds the following unit tests OR does not need testing for this extremely good reason:

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Documentation

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[codecov-io]

Codecov Report

Merging #6674 into master will decrease coverage by 0.3%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #6674      +/-   ##
==========================================
- Coverage   83.82%   83.52%   -0.31%     
==========================================
  Files         668      668              
  Lines       37543    37543              
==========================================
- Hits        31472    31356     -116     
- Misses       6071     6187     +116

Impacted Files	Coverage Δ
airflow/gcp/utils/field_validator.py	92.3% <ø> (ø)	⬆️
airflow/contrib/operators/file_to_wasb.py	100% <ø> (ø)	⬆️
airflow/jobs/backfill_job.py	91.43% <ø> (ø)	⬆️
airflow/operators/dagrun_operator.py	96% <100%> (ø)	⬆️
airflow/models/taskinstance.py	93.46% <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/refresh_config.py	50.98% <0%> (-23.53%)	⬇️
...rflow/contrib/operators/kubernetes_pod_operator.py	78.2% <0%> (-20.52%)	⬇️
... and 2 more

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[mppetkov]

Hello, are you sure that this PR is related to https://issues.apache.org/jira/browse/AIRFLOW-6062 ? The code changes seem to be different. Thanks!

[ashb]

Yup, should have been AIRFLOW-6080. I have updated.

[houqp]

My bad, copy pasted the wrong id. Thanks @ashb for fixing it :)