Drop support for EOL Python 2.6

[hugovk]

Python 2.6 and 3.3 are EOL and no longer receiving security updates (or any updates) from the core Python team.

They're also little used.

Here's the pip installs for flask-cors from PyPI for July 2018:

python_version	percent	download_count
2.7	53.26%	122,313
3.6	37.41%	85,914
3.5	6.09%	13,990
3.7	1.71%	3,937
3.4	1.46%	3,347
3.3	0.04%	99
3.2	0.01%	17
2.6	0.01%	15
Total		229,632

Source: pypinfo --start-date 2018-07-01 --end-date 2018-07-31 --percent --markdown flask-cors pyversion

Python 3.3 was recently dropped in https://github.com/corydolphin/flask-cors/commit/0fc5ebe60adbe6706e994eaa39ab0df096507516, this PR drops 2.6 (which reached EOL in 2013) and allows modern features of Python to be used. A couple of code inspections things are fixed as well.

Closes #232.