Stop using simplejson and use the standard library instead

[FlorentClarret]

What does this PR do?

Stop using simplejson and use the standard library instead

Motivation

• We used to use it for performance reason with Python 2. Now py2 is long gone, we can switch to the standard library.

Note: I'm not dropping the library (yet) because we need to switch to json in the other repos (namely: extras, marketplace and internal). I'll do that in a follow-up PR

Review checklist (to be filled by reviewers)

• Feature or bugfix MUST have appropriate tests (unit, integration, e2e)
• 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

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.28%. Comparing base (a19c5f3) to head (aed4700).
⚠️ Report is 974 commits behind head on master.

Additional details and impacted files

Flag	Coverage Δ
activemq	?
cassandra	?
ceph	∅ <100.00%> (?)
glusterfs	∅ <100.00%> (∅)
hdfs_datanode	∅ <100.00%> (∅)
hdfs_namenode	∅ <100.00%> (∅)
hive	?
hivemq	?
hudi	?
ignite	?
jboss_wildfly	?
kafka	?
kong	∅ <100.00%> (∅)
mapreduce	∅ <100.00%> (∅)
nginx	∅ <100.00%> (∅)
openstack	∅ <100.00%> (∅)
openstack_controller	∅ <100.00%> (?)
presto	?
riak	∅ <100.00%> (∅)
riakcs	∅ <100.00%> (∅)
solr	?
spark	∅ <100.00%> (∅)
twemproxy	∅ <100.00%> (∅)
vault	∅ <100.00%> (?)

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[sarah-witt]

Awesome! Can we also update these, either in this PR or a separate one?

https://github.com/DataDog/integrations-core/blob/master/spark/datadog_checks/spark/spark.py#L8

https://github.com/DataDog/integrations-core/blob/master/vault/datadog_checks/vault/vault.py#L15-L18

[FlorentClarret]

Awesome! Can we also update these, either in this PR or a separate one?

master/spark/datadog_checks/spark/spark.py#L8

master/vault/datadog_checks/vault/vault.py#L15-L18

I fixed them all, some other were missing, sorry about that