Remove 3.8 azure pipelines jobs
Remove 3.8 azure pipelines jobs

The azure pipelines environments do not have python 3.8 yet. Instead of
continuously waiting for this to show up lets just rely on travis for
3.8 testing for now and add windows and macOS coverage when it's
available.

masayukig/stestr,mtreinish/stestr,masayukig/stestr,mtreinish/stestr