Drop support for EOL Python 2.7 and 3.5

[hugovk]

Python 2.7 and 3.5 have reached end-of-life:

Branch	Schedule	First release	End-of-life
3.5	PEP 478	2015-09-13	2020-09-30
2.7	PEP 373	2010-07-03	2020-01-01

https://devguide.python.org/devcycle/#end-of-life-branches

This PR drops support for them, removes old compatibility shims and backports, and upgrades code to use modern Python syntax.

[codecov-commenter]

Codecov Report

Merging #50 (0ed3ae5) into master (6758ac6) will increase coverage by 0.85%.
The diff coverage is 87.50%.

@@            Coverage Diff             @@
##           master      #50      +/-   ##
==========================================
+ Coverage   88.92%   89.78%   +0.85%     
==========================================
  Files          15       15              
  Lines        1626     1596      -30     
  Branches      309      309              
==========================================
- Hits         1446     1433      -13     
+ Misses        124      108      -16     
+ Partials       56       55       -1     

Impacted Files	Coverage Δ
humanfriendly/terminal/html.py	82.87% <60.00%> (+0.23%)	⬆️
humanfriendly/terminal/__init__.py	74.42% <66.66%> (ø)
humanfriendly/terminal/spinners.py	86.30% <80.00%> (ø)
humanfriendly/case.py	84.21% <84.61%> (+2.24%)	⬆️
humanfriendly/testing.py	93.72% <93.75%> (+0.89%)	⬆️
humanfriendly/__init__.py	92.82% <100.00%> (ø)
humanfriendly/compat.py	90.90% <100.00%> (+27.75%)	⬆️
humanfriendly/deprecation.py	95.65% <100.00%> (ø)
humanfriendly/prompts.py	96.26% <100.00%> (-0.04%)	⬇️
humanfriendly/tables.py	92.63% <100.00%> (ø)
... and 2 more

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[hugovk]

Closing year-old PR due to lack of interest, but happy to re-open.

---

Here's the pip installs for humanfriendly from PyPI for December 2021, showing low numbers for 2.7 and 3.5:

category	percent	downloads
3.7	35.06%	2,503,508
3.6	27.88%	1,991,080
3.8	24.67%	1,761,824
3.9	7.46%	532,900
null	2.01%	143,780
2.7	1.64%	116,946
3.10	0.93%	66,113
3.5	0.35%	24,694
3.4	0.01%	517
3.11	0.00%	40
Total		7,141,402

Date range: 2021-12-01 - 2021-12-31

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