…d names

Add [flake8](http://flake8.pycqa.org) tests to find Python syntax errors and undefined names.

__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

exception (lowercase 'e') is an undefined name in this context while Exception (uppercase 'e') is the superclass.

flake8 testing of https://github.com/cclauss/pythonparser on Python 2.7.14

$ flake8 . --count --select=E901,E999,F821,F822,F823 --show-source --statistics

./pythonparser/source.py:259:9: F821 undefined name 'exception'
        exception.__init__(self, "Ranges %s and %s overlap" % (repr(first), repr(second)))
        ^
1     F821 undefined name 'exception'
1

Also follow Python porting best practice [__use feature detection instead of version detection__](https://docs.python.org/3/howto/pyporting.html#use-feature-detection-instead-of-version-detection).

* https://devguide.python.org/#branchstatus
* travis-ci/travis-ci#9069

Travis CI: flake8 tests to find Python syntax errors & undefined names

This allows Python2 code to inherit the parser without monkeypatches

As in `print >> self.stdout` with no value

https://m-labs.hk/pythonparser is 404