Drop support for Python 2.

[graingert]

To all users of shipwright, and @ all contributors/and people with forks:

Supporting both Python 2 and Python 3 is becoming a maintenance burden, and blocking use of the shiny new Python 3 features.

• nice mypy type annotations
• async/await
• yield from
• f'strings'

I'd like to move shipwright Python 3.x only and would like to gather your input.

• Do you use Shipwright under Python 2.x?
• Would you be unable to move to Python 3.x
• What's the latest Python you'd be able to support? 3.4, 3.5, 3.6?
• What sort of time-line would be appropriate:
  • 🎉 ASAP
  • 😄 March 2017 (next month)
  • 👍 Dec 2017 (match Django)
  • 👎 Jan 2020 (match CPython 2.7)
  • 😕 Never (match PyPy)

@cwoodcock
@danpalmer
@dayer4b
@dilgerma
@dropcam
@graingert
@jamespic
@jbwinters
@kaeawc
@likhvar
@nigelgbanks
@partkyle
@pierre-moire
@pombredanne
@srobertson
@tescalada
@timtylin

[jamespic]

Dropping support for Python 2 would not be a big problem. The way we're using it, sudo pip uninstall shipwright; sudo pip3 install shipwright would work just fine, since we don't call it from anything other than shell scripts.

As for which Python 3, we've still got significant stuff on Ubuntu 14.04, so anything over 3.4 is likely to cause headaches. Sorry. I know how neat it would be to use f'strings'.

[pombredanne]

This is always a difficult choice. My take would be to match Django. and keep supporting both until then.
When you support Python3, IMHO anything below 3.5 is not worth it. And by them 3.6 and up may be a best choice.

[srobertson]

We use it as standalone tool, not a library so moving support to python 3 wouldn't be too painful.