Travis CI: Use flake8 to find Python syntax errors or undefined names

CLA: trivial

In Travis CI, add a Python linting step that runs flake8 tests in Travis CI
to find syntax errors and undefined names. (http://flake8.pycqa.org

)

__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.

* 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

Reviewed-by: Paul Dale <paul.dale@oracle.com>
Reviewed-by: Matthias St. Pierre <Matthias.St.Pierre@ncp-e.com>
(Merged from https://github.com/openssl/openssl/pull/7410)