[Wheel-builders] Pre-release Python on docker image?

[Matthew Brett]

Hi,

New Python versions are a real problem for our current wheel system.
When Python 3.5 came out, suddenly the default Python installed from
Python.org did not have any binary wheels ready for it, so an install
of scipy on OSX went from a few seconds to some very long time or a
crash if you didn't have the requirements.

In order to avoid that, we'll need to make it easy to build and ship
wheels for upcoming Python releases.

I just implemented that for OSX / travis-ci over at
https://github.com/matthew-brett/multibuild - you can add e.g. Python
version string `3.6.0a3` to build against, which downloads and
installs the pre-release installer for OSX.

What to do about manylinux?   I guess we need to have a pre-release
Python built on the docker image.

Cheers,

Matthew