Drop Python 2.7 support by 2020

[bbayles]

It's almost 2019, which will be the last year with any releases that explicitly support Python 2.7. See here for some information on motivation.

What this means:

• I'll continue to accept additions that require Python 2-specific code throughout 2019
• At the end of 2019 I'll make a final Python 2.7-compatible release
• After that release I will make issues for making use of Python 3-only features, like yield from
• I'll remove six from the requirements and remove the places where it's used
• I'll accept PRs that change existing tools to use Python 3-only features
• I'll accept PRs that add new tools with Python 3-only features

If someone wants to make a fork targeting older Python versions they are of course welcome to; if requested I will provide a link in the docs.

[jaraco]

I'm strongly in favor of dropping support for Python 2.7. I'd even be in support of a more aggressive timeline, but accepting bugfix releases against older versions for the next year.

[bbayles]

If now is good enough for numpy, it's good enough for me. I'll open a PR to drop Python 2-isms - let me know if you see any I missed.

Anybody have any opinions on using yield from instead of return, for tools that give back an iterator? For example, this function returns, but could yield from instead.

[jaraco]

I would prefer return where suitable unless you do in fact want the deferral of behavior preceding the yield from.

[erikrose]

I generally reach for ‘yield from’ only when it can eliminate a ’for’ loop.

[hugovk]

🎆#268 has been merged.

Should this issue be closed, or kept open for visibility?

[bbayles]

Ah, yes. I'll close this when I merge #271 and make a release.

[bbayles]

And it's out. Many thanks!