Skip auto-setting Python version suffix if using an RC build

[florimondmanca]

What does this PR do?

Skip adding the -py2 or -py3 suffix if we're using an RC Agent build, such as datadog/agent:7.23.0-rc.2.

Motivation

RC builds aren't published with a -py2 or -py3 suffix, and I got tired of commenting out this auto-set code when doing QA. :-)

Additional Notes

Might need an additional tweak and thinking, because even public tags don't have -py2 or -py3 suffixes either, but only internal dev versions such as datadog/agent-dev:master-py3. But for now I'm just scratching my own itch!

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 #7653 into master will decrease coverage by 7.68%.
The diff coverage is n/a.

Impacted Files	Coverage Δ
mapr/datadog_checks/mapr/common.py
riak/datadog_checks/riak/__about__.py
system_swap/tests/test_system_swap.py
tcp_check/datadog_checks/tcp_check/__about__.py
...er/datadog_checks/kube_metrics_server/__about__.py
mesos_slave/tests/conftest.py
couch/datadog_checks/couch/__about__.py
ssh_check/tests/test_integration.py
...adog_checks/cloud_foundry_api/cloud_foundry_api.py
mapr/tests/common.py
... and 750 more