Clean up unsupported Python versions

[int19h]

We no longer support Python <=3.7. Where PTVS code has Python version checks that are always false, we should remove the branches that are never taken, and any associated helpers etc. We should also revisit cases where we previously had a complicated implementation solely to support older Python versions and see if they can be simplified now.

This also applies to helpers written in Python, such as the test runner.