Move the Python 3 lint-y job from TravisCI to another CI provider

[exarkun]

This is part of an overall goal to stop using TravisCI entirely.

The TravisCI job represented by this matrix item:

 - os: linux
    python: '3.5'
    env: T=py35

Gives us some static-ish checks on Python 3 (note also that ​https://github.com/tahoe-lafs/tahoe-lafs/pull/734, currently in progress, switches it from Python 3.5 to Python 3.8).

These checks aren't run anywhere else on our CI system. Set up a new job on one of our more preferred CI providers that performs this task and delete the corresponding TravisCI configuration.