Drop python 2.6 support

[nitely]

Fixes #24

unidecode package no longer supports python 2.6.

A side effect of this is being able to do io.open('README.md', encoding='utf-8').read() in the setup.py file

[hugovk]

pip install from PyPI for last month show virtually nothing from 2.6:

$ pypinfo --percent --pip unicode-slugify pyversion
python_version percent download_count
-------------- ------- --------------
2.7              60.2%         11,086
3.5              13.9%          2,557
3.6              13.0%          2,404
3.4              12.8%          2,367
2.6               0.0%              9
3.7               0.0%              6
3.3               0.0%              1

How about also dropping 3.3? It's been EOL since last month.

[davedash]

@nitely can you bump the minor version for this?

[nitely]

@davedash done 👌

[nitely]

@davedash btw PR #22 won't fix the issue for Python 2.7. I can implement the io.open('README.md', encoding='utf-8').read() solution if you want me to, or you can do it later.

[davedash]

@nitely if you have the time, please do.