Drop support for old Python and pytest

[hugovk]

Python 3.4

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 pytest-factoryboy from PyPI for September 2019:

category	percent	downloads
3.7	45.44%	17,493
3.6	31.35%	12,071
2.7	17.05%	6,563
3.5	4.80%	1,847
null	1.20%	462
3.4	0.16%	60
2.6	0.01%	3
Total		38,499

Date range: 2019-09-01 - 2019-09-30

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

pytest 3.6-4.5, 5.0

Also pytest 4.0-4.2 are failing on master:

• https://travis-ci.org/hugovk/pytest-factoryboy/builds/596168409

Remove all the old ones and only test against pytest 4.6 (the last to support Python 2.7) and the latest version (currently 5.2).

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This change is 

[youtux]

Also please add an entry in CHANGES.rst

[youtux]

Please add an entry in CHANGES.rst

[hugovk]

Done!

[youtux]

Thank you!

[hugovk]

You're welcome!