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

They're also little used. Here's the pip installs for pytest-django from PyPI for August 2019:

category	percent	downloads
3.6	39.15%	250,673
3.7	35.13%	224,890
2.7	16.93%	108,415
3.5	7.24%	46,374
3.4	0.86%	5,495
null	0.52%	3,357
3.8	0.15%	973
2.6	0.01%	64
Total		640,241

Date range: 2019-08-01 - 2019-08-31

Source: pip install -U pypistats && pypistats python_minor pytest-django --last-month

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What are the plans for dropping Python 2.7? Here's the status of pytest and Django:

• https://pytest.org/en/latest/py27-py34-deprecation.html
• https://www.djangoproject.com/download/

[blueyed]

Thanks!

It does not allow us to remove code (but maybe the coverage report shows something), so I'd rather postpone this - since it would disallow install on py34 for no reason.

[blueyed]

Rebased (for up-to-date coverage report).
Dropping Django < 1.11 soon (#796), so this goes kind of in the same direction.