Drop support for EOL Python 2.6 and 3.3

[hugovk]

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

Version	Release date	Supported until
2.6	2008-10-01	2013-10-29
3.3	2012-09-29	2017-09-29

They're also relatively little used. Here are the downloads from PyPI for docker-pycreds for last month:

category	percent	downloads
2.7	66.44%	1,480,078
3.6	22.85%	509,075
3.5	7.16%	159,472
3.7	1.69%	37,606
3.4	1.30%	28,911
null	0.43%	9,538
2.6	0.12%	2,683
3.8	0.01%	118
3.3	0.00%	65
3.2	0.00%	2
Total		2,227,548

[shin-]

Thank you!