Drop support for EOL Python 2.7 and 3.5

[hugovk]

And upgrade Python syntax with pyupgrade --py36-plus

Here's the pip installs for livereload from PyPI for July 2021:

category	percent	downloads
3.7	28.83%	176,847
3.9	26.51%	162,595
3.6	15.33%	94,056
3.8	15.32%	93,983
null	11.35%	69,644
2.7	1.96%	12,005
3.5	0.69%	4,214
3.10	0.01%	66
3.4	0.01%	35
3.11	0.00%	2
Total		613,447

Date range: 2021-07-01 - 2021-07-31

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