Drop support for EOL Python 2.7 and 3.5

[hugovk]

Fixes #1604.
Fixes #1596.

Done from #1604:

• Run pyupgrade --py36-plus and flynt on all the code
• Remove dependency on six and ipaddress packages
• Update python_requires in setup.py to be >=3.6
• Remove outdated classifiers from setup.py
• Remove outdated versions from continuous integration configuration
• Remove mentions of Python 2.7, 3.3, 3.4, 3.5 from docs/comments in the code
• Bump version on main to be (8, 0, 0)

Also:

• Remove unused mock backport (#1596)
• Simplify collections.abc import
• Remove universal wheel, only needed to support Python 2 and 3
• Update Black target version

Not done from #1604:

• Remove "required commit statuses" from the main branch protection rules -> admin task

[sethmlarson]

LGTM, thanks @hugovk!