Upgrade default Python from 3.6 on Trusty to 3.8 on Xenial

[cclauss]

A continuation of #1678 now that dist: xenial is the Travis CI default we can upgrade from Python 3.6 to 3.7 as the default Python on Travis CI Linux jobs.

Python 3.7 has a slightly stricter syntax than 3.6 because async and await are keywords in Python >= 3.7. For projects that use Travis CI with the default Python, this pull request will allow them to find these issues before their users do.

NOTE: This pull request currently comments out one test which is run on the Trusty-based travisci/ci-garnetcontainer because Python 3.7 does not run on Trusty. Please let me know if this is acceptable or provide guidance on how to run that test on Xenial or later.

[cclauss]

@BanzaiMan @MariadeAnton Can anyone please help me to understand the failure message?
https://travis-ci.org/travis-ci/travis-build/jobs/558975609#L746

Now that we are running on xenial, I believe that we need to do the equivalent of:

addons:
  apt:
    sources:
      - docker-xenial
    packages:
      - docker

https://docs.travis-ci.com/user/reference/xenial/#third-party-apt-repositories-removed

I can apt_packages: ['docker'], but setting apt sources eludes me. @native-api

[native-api]

I believe docker is preinstalled, no need to install anything.
https://docs.travis-ci.com/user/docker/
https://travis-ci.org/native-api/test_travis/builds/565893910

And I really don't see why this PR attempts to add it.

[cclauss]

https://docs.travis-ci.com/user/reference/xenial/#third-party-apt-repositories-removed

[native-api]

By "I don't see why", I meant because Docker is completely irrelevant to Python's default version thus to this PR's topic.

[cclauss]

Our only failing test is Travis::Build::Script::Python behaves like a bash script full docker container execution runs and passes. Do you believe this is not a useful test?

[native-api]

Docker tests are run in travisci/ci-garnet container which is Trusty-based. And 3.7 is not available for Trusty.

[cclauss]

Docker tests are run in travisci/ci-garnet container which is Trusty-based. And 3.7 is not available for Trusty.

Thanks @native-api Where would I fix that test given that Trusty is EOL?