Update supported Python versions

[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 atomicwrites from PyPI for June 2018:

python_version	percent	download_count
3.6	72.14%	878,675
2.7	19.73%	240,284
3.5	5.87%	71,478
3.4	1.92%	23,405
3.7	0.30%	3,703
3.3	0.03%	354
2.6	0.01%	139
3.8	0.00%	31
3.2	0.00%	8
None	0.00%	6
Total		1,218,083

Source: pypinfo --start-date 2018-06-01 --end-date 2018-06-30 --percent --markdown atomicwrites pyversion

This also adds Trove classifiers, and python_requires to help pip.

[untitaker]

Thanks!