Drop support for EOL Python 3.4

[hugovk]

Python 3.4 is EOL and no longer receiving security updates (or any updates) from the core Python team.

Version	Release date	Supported until
2.7	2010-07-03	2020-01-01
3.4	2014-03-16	2019-03-16

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

It's also little used. Here's the pip installs for prettytable from PyPI for July 2019:

category	percent	downloads
2.7	78.60%	1,380,358
3.6	10.89%	191,172
3.7	5.10%	89,496
3.5	4.53%	79,487
3.4	0.56%	9,875
null	0.29%	5,045
3.8	0.02%	373
2.6	0.01%	236
3.3	0.00%	36
3.1	0.00%	2
3.9	0.00%	1
Total		1,756,081

Source: pip install -U pypistats && pypistats python_minor prettytable --last-month

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@d5427b3). Click here to learn what that means.
The diff coverage is 100%.

@@            Coverage Diff            @@
##             master      #16   +/-   ##
=========================================
  Coverage          ?   76.25%           
=========================================
  Files             ?        1           
  Lines             ?      998           
  Branches          ?        0           
=========================================
  Hits              ?      761           
  Misses            ?      237           
  Partials          ?        0

Impacted Files	Coverage Δ
prettytable.py	76.25% <100%> (ø)

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