Check failing flake tests

[santosomar]

Run # stop the build if there are Python syntax errors or undefined names
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./openVulnQuery/_library/_compatibility.py:2:23: F821 undefined name 'basestring'
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string_or_bytes = basestring
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^
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1 F821 undefined name 'basestring'

[santosomar]

Also several tests were failing because of the mock module. In newer versions of Python from unittest import mock should be used instead. Thank you @mschuele2