[tests] Remove Python 3.5 from unit-tested versions

[sgnn7]

What does this PR do?

Remove Python 3.5 from the unit test matrix

Description of the Change

Azure builders do not have this Python version anymore so they cannot run against it. Given that 3.5 is EOL we will remove it from the test matrix.

Alternate Designs

Possible Drawbacks

Python 3.5 support may get stale over time

Verification Process

CI/CD passes

Additional Notes

Release Notes

Review checklist (to be filled by reviewers)

• [ ] Feature or bug fix MUST have appropriate tests (unit, integration, etc...)
• [ ] 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 one changelog/ label attached. If applicable it should have the backward-incompatible label attached.
• [ ] PR should not have do-not-merge/ label attached.
• [ ] If Applicable, issue must have kind/ and severity/ labels attached at least.

[therve]

/azp run

[azure-pipelines[bot]]

Azure Pipelines successfully started running 2 pipeline(s).

[sgnn7]

Failing tests are known flakes. Merging.