Switch to Python 3.8 in check integration template

[florimondmanca]

What does this PR do?

Update the check template so that it uses Python 3.8 for Tox environments, instead of Python 3.7.

Motivation

We shouldn't be using Python 3.7 anymore, because we moved to 3.8. See: #5626.

Additional Notes

Discovered while working on #5715.

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 #5717 into master will decrease coverage by 3.82%.
The diff coverage is n/a.

Impacted Files	Coverage Δ
...ts/tooling/configuration/consumers/test_example.py	100% <0%> (ø)	⬆️
...checks_dev/tests/plugin/test_environment_runner.py	100% <0%> (ø)	⬆️
datadog_checks_dev/datadog_checks/dev/warn.py	63.63% <0%> (ø)	⬆️
datadog_checks_dev/tests/common.py	100% <0%> (ø)	⬆️
...hecks_dev/tests/tooling/configuration/test_load.py	100% <0%> (ø)	⬆️
datadog_checks_dev/datadog_checks/dev/terraform.py	0% <0%> (ø)	⬆️
...tadog_checks/dev/tooling/configuration/template.py	100% <0%> (ø)	⬆️
...cks/dev/tooling/configuration/consumers/example.py	100% <0%> (ø)	⬆️
datadog_checks_dev/tests/test_docker.py	100% <0%> (ø)	⬆️
...v/tests/tooling/config_validator/test_validator.py	100% <0%> (ø)	⬆️
... and 886 more