Drop support for EOL Python 2.6 and 3.3

[hugovk]

Fixes #206.

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 fakeredis from PyPI for July 2018:

python_version	percent	download_count
2.7	43.69%	15,102
3.6	30.85%	10,661
3.5	14.93%	5,160
3.4	9.40%	3,249
3.7	1.06%	368
2.6	0.05%	17
3.3	0.02%	6
Total		34,563

Source: pypinfo --start-date 2018-07-01 --end-date 2018-07-31 --percent --markdown fakeredis pyversion

[coveralls]

Coverage remained the same at 98.157% when pulling 70a2bad on hugovk:rm-eol into aed0c2f on jamesls:master.

[bmerry]

Thanks for doing the work. I'm not going to merge it now because it makes fakeredis strictly less useful (even if it's only to a few people). However, I'll leave it open and if somebody makes a contribution that can't trivially support 2.6 then it can be revisited.

[bmerry]

I've removed 2.6 and 3.3 support in 1.0b1 (using this PR as a reference, although due to the extensive rewriting I didn't directly apply it), so closing.