Added APE describing roadmap for ending Python 2 support (APE10)

[astrofrog]

The purpose of this APE is to set out a roadmap for future releases of the Astropy core package, in particular to identify the process for switching to releases that support only Python 3 and ending support for Python 2.7 in December 2019.

If you would like to leave comments on this, please aim to do so in the next couple of weeks, and by August 19th at the very latest.

Rendered version: https://github.com/astrofrog/astropy-APEs/blob/ape-python-3/APE10.rst

Note: We can settle on whether this is APE 9 or APE 10 once it is ready to merge

[mwcraig]

Looks good to me. I prefer the earlier transition to the later one mentioned as an alternative.

[mhvk]

Looks sensible. I also prefer the earlier transition.

[Cadair]

👍

[astrofrog]

@eteq @kelle - the section on pip has been re-written in light of recent improvements/progress on the pip side, and also to mention that we can re-assess things later: https://github.com/astrofrog/astropy-APEs/blob/ape-python-3/APE10.rst#packages-on-pypi

[eteq]

Thus far the ordering of APEs has been set by the ordering of pull requests. So given this came in after #12, #12 is APE9 and this is APE10.

[eteq]

Also, the coordination committee has discussed this (with @astrofrog recusing due to being the primary author), and it was agreed this APE should be accepted. I will merge this shortly (with updates for the date and desciption of discussion)