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

Source: https://en.wikipedia.org/wiki/CPython#Version_history

They're also little used.

Here's the pip installs for configobj from PyPI for September 2018:

python_version	percent	download_count
2.7	76.56%	607,597
3.4	13.42%	106,487
3.6	5.52%	43,824
3.7	2.84%	22,520
3.5	1.54%	12,231
2.6	0.11%	875
3.3	0.01%	65
3.2	0.00%	6
3.8	0.00%	2
Total		793,607

Source: pypinfo --start-date 2018-09-01 --end-date 2018-09-30 --percent --markdown configobj pyversion

Finally, they're failing the build (see PR #172, https://travis-ci.org/DiffSK/configobj/builds/440978367) due to other dependencies having already dropped them, as the Python community is generally doing.

[coveralls]

Coverage remained the same at 81.463% when pulling 2b69f97 on hugovk:rm-eol into 5b5de48 on DiffSK:master.

[jhermann]

Included in #174