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).

Author: cclauss

Diff for: .lgtm.yml

@@ -1,26 +1,5 @@
 language: python
-cache: pip
-python:
-    - 2.7
-    - 3.6
-    #- nightly
-    #- pypy
-    #- pypy3
-matrix:
-    allow_failures:
-        - python: nightly
-        - python: pypy
-        - python: pypy3
-install:
-    #- pip install -r requirements.txt
-    - pip install flake8  # pytest  # add another testing frameworks later
-before_script:
-    # stop the build if there are Python syntax errors or undefined names
-    - flake8 . --count --select=E9,F63,F72,F82 --show-source --statistics
-    # exit-zero treats all errors as warnings.  The GitHub editor is 127 chars wide
-    - flake8 . --count --exit-zero --max-complexity=10 --max-line-length=127 --statistics
-script:
-    - true  # pytest --capture=sys  # add other tests here
-notifications:
-    on_success: change
-    on_failure: change  # `always` will be the setting once code changes slow down
+dist: xenial  # required for Python >= 3.7
+python: 3.7
+install: pip install flake8
+script: flake8 . --count --select=E9,F63,F7,F82 --show-source --statistics