Drop support for EOL Python

[hugovk]

Python 2.4-2.6 are EOL (since at least 2008, 2011 and 2013 respectively) 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 prettytable from PyPI for August 2018:

python_version	percent	download_count
2.7	88.65%	1,064,121
3.6	6.53%	78,341
3.5	2.55%	30,639
3.4	1.30%	15,582
3.7	0.91%	10,976
2.6	0.04%	471
3.3	0.02%	185
3.8	0.00%	15
Total		1,200,330

Source: pypinfo --start-date 2018-08-01 --end-date 2018-08-31 --json prettytable pyversion

This PR drops support and upgrades some code for newer Python versions.