Drop support for Python 2.6 and 3.0-3.4

[hugovk]

Python 2.6 and 3.0-3.2 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
3.0	2008-12-03	2009-06-27
3.1	2009-06-27	2012-04-09
3.2	2011-02-20	2016-02-27

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

They're also little used. Here's the pip installs for humanfriendly from PyPI for November 2018:

category	percent	downloads
3.6	71.82%	794,039
2.7	14.47%	159,943
3.5	10.15%	112,226
3.7	2.25%	24,878
3.4	0.98%	10,820
null	0.20%	2,223
2.6	0.13%	1,410
3.3	0.01%	74
3.8	0.00%	3
Total		1,105,616

Source: pypistats python_minor --last-month humanfriendly

This also fixes some Flake8 errors, upgrades Python syntax with pyupgrade, and adds python_requires to help pip install the relevant package for the running Python.

[coveralls]

Coverage increased (+0.1%) to 95.111% when pulling 3030450 on hugovk:rm-eol into 03d1db4 on xolox:master.

[and-semakin]

Should be merged 👍

[hugovk]

Updated to drop support for Python 3.4, EOL since 2019-03-16.

Here's the pip installs for June 2019:

category	percent	downloads
3.6	36.64%	344,300
3.7	32.63%	306,597
2.7	22.07%	207,388
3.5	7.71%	72,425
3.4	0.48%	4,470
null	0.41%	3,825
2.6	0.06%	576
3.8	0.01%	53
3.3	0.00%	13
Total		939,647

Source: pypistats python_minor humanfriendly --last-month # pip install pypistats

[xolox]

This has been merged, thanks Hugo!