Travis CI: Simplify the testing

.travis.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