Remove use of 'six' package from most tests (and haproxy and istio)

[iliakur]

What does this PR do?

Motivation

Started out by removing six from HA Proxy and Istio, then realized it's more incremental to remove it from tests first. So then I focused on that.

Additional Notes

Review checklist (to be filled by reviewers)

• Feature or bugfix MUST have appropriate tests (unit, integration, e2e)
• Changelog entries must be created for modifications to shipped code
• Add the qa/skip-qa label if the PR doesn't need to be tested during QA.
• If you need to backport this PR to another branch, you can add the backport/<branch-name> label to the PR and it will automatically open a backport PR once this one is merged

[codecov]

Codecov Report

Attention: Patch coverage is 97.52066% with 3 lines in your changes missing coverage. Please review.

Project coverage is 90.19%. Comparing base (35be561) to head (3d5f8d1).
Report is 1 commits behind head on master.

Additional details and impacted files

Flag	Coverage Δ
activemq	?
amazon_msk	88.84% <100.00%> (ø)
cassandra	?
clickhouse	94.53% <0.00%> (+0.36%)	⬆️
consul	91.81% <100.00%> (-0.01%)	⬇️
datadog_checks_base	89.00% <100.00%> (+0.31%)	⬆️
datadog_checks_dev	77.31% <100.00%> (ø)
disk	90.75% <100.00%> (+1.40%)	⬆️
dns_check	93.29% <100.00%> (-0.04%)	⬇️
elastic	93.34% <100.00%> (-0.01%)	⬇️
esxi	93.23% <100.00%> (+0.17%)	⬆️
go_expvar	92.69% <100.00%> (-0.05%)	⬇️
haproxy	95.11% <92.30%> (+0.15%)	⬆️
hdfs_datanode	89.69% <100.00%> (-0.06%)	⬇️
hdfs_namenode	86.66% <100.00%> (-0.07%)	⬇️
hive	?
hivemq	?
ibm_mq	91.25% <100.00%> (+0.11%)	⬆️
ignite	?
istio	77.86% <100.00%> (+0.34%)	⬆️
jboss_wildfly	?
kafka	?
kubelet	91.01% <100.00%> (-0.01%)	⬇️
lighttpd	83.64% <100.00%> (ø)
mapr	82.35% <100.00%> (-0.08%)	⬇️
mapreduce	82.03% <100.00%> (-0.05%)	⬇️
mesos_master	89.75% <100.00%> (-0.07%)	⬇️
mesos_slave	93.26% <100.00%> (-0.05%)	⬇️
network	93.03% <100.00%> (+0.42%)	⬆️
openstack	55.15% <100.00%> (-0.05%)	⬇️
openstack_controller	94.46% <100.00%> (+0.02%)	⬆️
postfix	88.04% <100.00%> (-0.07%)	⬇️
postgres	93.06% <100.00%> (+3.40%)	⬆️
presto	?
process	85.59% <100.00%> (-0.03%)	⬇️
redisdb	87.76% <100.00%> (-0.02%)	⬇️
snmp	95.71% <0.00%> (ø)
solr	?
system_core	92.59% <100.00%> (-0.07%)	⬇️
tls	91.40% <100.00%> (-0.01%)	⬇️
vault	95.19% <100.00%> (+0.47%)	⬆️
vsphere	96.15% <100.00%> (-0.01%)	⬇️
yarn	89.50% <100.00%> (-0.03%)	⬇️
zk	82.15% <100.00%> (ø)

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