Travis CI: Add a more key flake8 tests

.travis.yml

@@ -3,13 +3,12 @@ language: python
 cache: pip
 python:
     - 2.7
-    - 3.6
+    - 3.7
 install:
-    - pip install -r requirements.txt
-    - pip install flake8  # pytest  # add another testing frameworks later
+    - pip install flake8 -r requirements.txt
 before_script:
     # stop the build if there are Python syntax errors or undefined names
-    - flake8 . --count --select=E901,E999,F821,F822,F823 --show-source --statistics
+    - flake8 . --count --select=E9,F63,F7,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: