Drop support for Python 3.3.

[h-hirokawa]

The End-of-life of Python 3.3 is already over, and Pytest >= 3.3.0 no longer supports 3.3.
To prevent build errors in Travis, it is better to stop supporting the version even in isort.

[hugovk]

Here's the pip installs for isort from PyPI for the last month (via pypinfo --percent --pip --markdown isort pyversion), with low numbers for 3.3:

python_version	percent	download_count
2.7	52.9%	314,042
3.6	25.0%	148,089
3.5	14.3%	84,643
3.4	6.9%	41,234
3.7	0.4%	2,159
3.3	0.3%	1,765
2.6	0.3%	1,503
3.2	0.0%	24
None	0.0%	2

[timothycrosley]

Agreed that it should be dropped; merge requests looks good!

Thanks!