CI: Integrate 3.8 builds

[TomAugspurger]

3.8 uses Python from Travis-CI.

Once conda packages are built for 3.8 we want to use our usual system.

[mohitanand001]

RemoveError: 'setuptools' is a dependency of conda and cannot be removed from conda's operating environment.

New commits to pull requests shows this error in Travis CI. Is this related.??

[TomAugspurger]

I doubt it.