Drop support for EOL Python 2.6 and 3.3

[hugovk]

Latest master is failing on Python 2.6 and 3.3 because pytest, or rather py, has dropped support for Python 3.3 (pytest-dev/py#159, pytest-dev/py#165):

https://travis-ci.org/hugovk/astroid/builds/309952810

Here's the pip installs for pytest-mock from PyPI for the last month (via pypinfo --percent --pip pytest-mock pyversion) showing a small fraction on 2.6 and 3.3:

python_version	percent	download_count
2.7	61.5%	115,963
3.6	17.4%	32,841
3.5	16.2%	30,459
3.4	3.5%	6,526
2.6	0.5%	963
3.3	0.5%	878
3.7	0.4%	808
3.2	0.0%	54

[nicoddemus]

Awesome @hugovk, thanks! 😁