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 deepdiff from PyPI for December 2019:

category	percent	downloads
2.7	65.98%	762,527
3.6	17.69%	204,486
3.7	9.98%	115,302
3.5	5.10%	58,941
3.8	0.88%	10,196
null	0.25%	2,920
3.4	0.12%	1,341
2.6	0.00%	5
3.9	0.00%	3
3.3	0.00%	1
Total		1,155,722

Date range: 2019-12-01 - 2019-12-31

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

This PR also upgrades Python syntax with https://github.com/asottile/pyupgrade/, caches pip on the CI to speed up builds, and removes some CI config for Python 3.8 which is no longer needed.

[seperman]

Thanks @hugovk
This is awesome! Didn't know about the stats. Very cool.

[seperman]

👍

[seperman]

That's kind of sad that 2.7 is still the majority of downloads. We even raise an exception for 2.7:

if py2:  # pragma: no cover
    sys.exit('Python 2 is not supported anymore. The last version of DeepDiff that supported Py2 was 3.3.0')