Drop support for EOL Python 2.7 and 3.4-3.5

[hugovk]

Fixes #717.

Follow on from #706 (comment).

Python 2.7 and 3.4 - 3.5 are EOL:

cycle	latest	release	eol
3.6	3.6.15	2016-12-23	2021-12-23
3.5	3.5.10	2015-09-30	2020-09-13
3.4	3.4.10	2014-03-16	2019-03-18
2.7	2.7.18	2010-07-03	2020-01-01

https://endoflife.date/python

Here's the pip installs for prometheus_client from PyPI for October 2021 showing under 2% usage:

category	percent	downloads
3.7	72.89%	22,490,512
3.8	14.20%	4,380,172
3.6	6.85%	2,113,567
3.9	2.93%	904,146
null	1.30%	402,541
2.7	1.30%	401,660
3.5	0.27%	82,532
3.10	0.26%	79,007
3.4	0.00%	736
3.11	0.00%	184
3.3	0.00%	2
Total		30,855,059

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

In fact, 2.7 + 3.4 + 3.5 usage combined is under 2% = 1.57%.

Also upgraded Python syntax with https://github.com/asottile/pyupgrade --py36-plus, and removed universal wheels, only needed for suporting both Python 2 and 3.

[csmarchbanks]

Thanks! This will close #717 (just making a reference for the future).

I posted that to the -dev mailing list yesterday, and am waiting to hear if there are significant concerns from the community before moving forward with review/merge on this. If I don't hear anything within a week at the most we can certainly move forward.

[csmarchbanks]

I haven't heard any concerns on the other issue, so I plan to review this sometime this week!

[csmarchbanks]

This looks awesome, thanks for all your work! I left a couple comments/questions, but they are pretty minor. I might try to get a second set of eyes to make sure I am not missing any python 2/3 conversion differences.

[csmarchbanks]

👍 from me, double checking with a second reviewer, then will merge. Thanks again!