Travis CI: Simplify the testing (#887)

* Travis CI: Simplify the testing

* flake8 . --count --select=E9,F63,F7,F82 --show-source --statistics

* python: 3.7

* dist: xenial for python: 3.7

* Delete .lgtm.yml

These changes were created to get around the fact that Travis CI was not enabled on this repo.  Now that Travis is enabled, we can remove these modifications.

The problems are that:
1. [LGTM does not want us running flake8 on their infrstructure](https://discuss.lgtm.com/t/can-i-get-lgtm-to-run-flake8-tests/1445/6) and
2. when we do, it [does not work as expected](https://discuss.lgtm.com/t/tests-are-not-automatically-run-when-lgtm-yml-is-modified/1446/4).