Remove support for Python < 3.6

[anoadragon453]

EDIT: Top comment hijacked by @callahad to track remaining work.

• Remove support for Python < 3.6
• Update CI to only test on 3.6+
• Update setup.py to state a minimum Python version of 3.6 (example from Sygnal)
• Update pyproject.toml to state a target Python version for Black (example from Synapse)
• Remove all mention and use of six, as we no longer support Python 2
• Remove all # -*- coding: utf-8 -*- lines from files; Python 3 is UTF-8 by default
• Run pyupgrade on Sydent with arguments --py36-plus --keep-percent-format

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Original Comment:

Sydent supports Python 3 as of a few months ago. However, it also still supports Python 2, which is now end of life. In order for Sydent to take advantage of newer Python features, support for Python 2 will eventually need to be dropped.

Some questions:

1. Do we need to announce deprecation? If so, how long should the transition period be?
2. Would removing Python 2 support negatively impact a significant population?
3. What does removing Python 2 support look like?

I bring this up as I'd like to start using some new language features, and only just realised that our official stance on Python 2 support was still in flux :)

[clokep]

We talked about this a bit today, some answers inline:

Do we need to announce deprecation? If so, how long should the transition period be?

I don't think so, I think the next release can drop Python 2 support. It might be worth doing a point release with any bugfixes, if there are any.

Would removing Python 2 support negatively impact a significant population?

It does not seem that anyone is still using Python 2 with sydent.

What does removing Python 2 support look like?

To start with, just updating setup.py with python_requires and classifiers to state this is Python 3 only. We can eventually clean-up any use of six / Python 2 compatible code but that isn't a requirement.

[richvdh]

plan is to announce in the v2.3.0 release that it will be the last to support python 2 or python 3.5

[richvdh]

This is now done. Next steps are to remove python2 and python 3.5 from CI, and update setup.py to require python 3.6.