Travis CI: Add automated tests to find syntax errors

[cclauss]

Use flake8 to look for Python syntax errors or undefined names on both Python 2.7 and Python 3.7.

E901,E999,F821,F822,F823 are the "showstopper" flake8 issues that can halt the runtime with a SyntaxError, NameError, etc. Most other flake8 issues are merely "style violations" -- useful for readability but they do not effect runtime safety. This PR therefore recommends a flake8 run of these tests on the entire codebase.

• F821: undefined name name
• F822: undefined name name in __all__
• F823: local variable name referenced before assignment
• E901: SyntaxError or IndentationError
• E999: SyntaxError -- failed to compile a file into an Abstract Syntax Tree

[cclauss]

https://travis-ci.org/apachecn would need to turned on like
https://travis-ci.org/apache is for these automated tests to run.