Drop support for EOL Python 2.6

[hugovk]

Fixes #47.

Python 2.6 is 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

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

It's also little used. Here's the pip installs for python-jsonrpc from PyPI for December 2018:

category	percent	downloads
2.7	95.80%	5,538
null	2.27%	131
3.6	1.11%	64
3.7	0.62%	36
3.5	0.14%	8
2.6	0.05%	3
3.4	0.02%	1
Total		5,781

Source: pypistats python_minor --last-month python-jsonrpc