Remove transient dependency pin

[ChristineTChen]

What does this PR do?

Add flag to pip install to resolve dependencies

Motivation

https://github.com/tobgu/pyrsistent/blob/97190f8ad43553603a95a83361d4c0fe02e86970/CHANGES.txt#L4-L5

Dropped py2 support resulting in CI failing https://dev.azure.com/datadoghq/integrations-core/_build/results?buildId=23293&view=logs&j=910f82a4-622d-5ace-f877-8b75c045486c&t=b8a76c45-01d6-50b1-e11b-d9fedb0dbe38&l=230

Additional Notes

Review checklist (to be filled by reviewers)

• Feature or bugfix MUST have appropriate tests (unit, integration, e2e)
• PR title must be written as a CHANGELOG entry (see why)
• Files changes must correspond to the primary purpose of the PR as described in the title (small unrelated changes should have their own PR)
• PR must have changelog/ and integration/ labels attached

[codecov]

Codecov Report

Merging #7545 into master will decrease coverage by 6.51%.
The diff coverage is n/a.

Impacted Files	Coverage Δ
kubelet/datadog_checks/kubelet/common.py
clickhouse/datadog_checks/clickhouse/clickhouse.py
...tem_swap/datadog_checks/system_swap/system_swap.py
..._controller/tests/test_nginx_ingress_controller.py
coredns/tests/test_check.py
disk/tests/test_unit.py
openldap/datadog_checks/openldap/openldap.py
system_core/datadog_checks/system_core/__init__.py
...y_api/datadog_checks/cloud_foundry_api/__init__.py
powerdns_recursor/tests/test_powerdns.py
... and 823 more