Drop support for Python 2.7

[jaraco]

In GitLab by @jaraco on Oct 19, 2020, 15:38

Python 2.7 is end-of-life. It may be time to just drop support for it.

However, once support is dropped, new features (including support for traversable providers) is unlikely to be supported. How important is that?

This compatibility is effectively blocking updates to CPython master to use f-strings throughout the codebase.

[jaraco]

My main reluctance in enacting this change forthwith is the issues in #90 and #183 would not reach Python 2.7. It looks like #183 would back-port easily enough, so I'm not going to worry too much about blocking on those and allow removal of 2.7 support at any point.

[jaraco]

Done in #208 and released as version 4.