Drop support for legacy Python 3.4

[hugovk]

Changes proposed in this pull request:

• It's not long until Python 3.4's EOL on 2019-03-16
• Python 3.4 isn't used much
• A backwards-incompatible release is coming up soon (dropping Python 2 Drop support for legacy Python 2 #281) so it's a good time to bundle this into a major version release
• Upgrade Python syntax with pyupgrade `find . -name "*.py"` --py3-plus
• Includes PR Drop support for legacy Python 2 #281 to avoid merge conflicts

Here's the pip installs for pylast from PyPI for last month:

category	percent	downloads
2.7	41.59%	2,123
3.6	36.85%	1,881
3.5	8.85%	452
null	5.99%	306
3.7	5.80%	296
3.4	0.82%	42
3.3	0.10%	5
Total		5,105

Source: pypistats python_minor --last-month pylast

https://github.com/hugovk/pypi-tools#examples

[hugovk]

PR failed due to #171 but passed on my fork:

https://travis-ci.org/hugovk/pylast/builds/459953101

[hugovk]

Closing in favour of #289, which can run tests and has also been rebased to remove merge conflicts.