[AIRFLOW-6575] Entropy source for CI tests is changed to unblocking

[potiuk]

On Travis CI blocking entropy source slows startup time of a number of
containers. This change changes the entropy source to unblocking one.

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Issue link: AIRFLOW-6575

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[potiuk]

Let's see when Travis completes, but I believe this change will speed up the startup - I changed the entropy source to be unblocking one and it should make the startup faster.

[ashb]

This looks like an acceptable long-term fix to me too.

[codecov-io]

Codecov Report

Merging #7185 into master will increase coverage by 0.12%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #7185      +/-   ##
==========================================
+ Coverage   85.25%   85.37%   +0.12%     
==========================================
  Files         723      723              
  Lines       39543    39543              
==========================================
+ Hits        33711    33761      +50     
+ Misses       5832     5782      -50

Impacted Files	Coverage Δ
airflow/jobs/scheduler_job.py	89.34% <0%> (+0.43%)	⬆️
airflow/hooks/dbapi_hook.py	91.73% <0%> (+1.65%)	⬆️
airflow/hooks/postgres_hook.py	94.36% <0%> (+16.9%)	⬆️
airflow/operators/postgres_to_gcs.py	85.29% <0%> (+32.35%)	⬆️
airflow/operators/postgres_operator.py	100% <0%> (+100%)	⬆️

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