Test rebuilds for Python transitions

This is an EXPERIMENTAL PPA. We recommend you do not use it for anything but testing purposes as incompatible changes and breaks may happen at any time.

This PPA is for test rebuilds during various stages of Python transitions in Ubuntu, such as Python 3.5 -> 3.6. Such transitions happen roughly every 18 months, or when a new upstream Python 3 version coincides with a new Ubuntu development series. We need to perform test rebuilds to gather information about the state of the Python ecosystem, before we begin uploads and rebuilds in the real archives. Python transitions usually go through these steps:

1) The new interpreter is uploaded to Ubuntu without making it a supported or default version, and without changing anything about the current default version of Python. This just gets the interpreter built and available in the archive, and while many pure-Python packages will be importable, we don't know anything about buildability. Also, no packages with extension modules will work, and no applications will use the new version.

2) Enable the new version as a supported version, leaving the current default as the default. We have to trigger rebuilds to make extension modules available, and at this point the new version should be generally usable.

3) Make the new version the default version, demoting the previous default version to a supported version. This step means that /usr/bin/python3 will point to the new version of Python and applications will generally use it by default, but you can still use the old version with explicit `pythonX.Y` calling syntax.

4) Drop support for the old version.

Steps 2 and 4 require archive rebuilds. Steps 2 and 3 are generally done in a PPA first. We analyze the results to estimate the amount of work that step in the transition is going to take. Step 2 usually means the most amount of work, as it can take a while for upstreams to officially support the new version. We estimate the amount of work involved for us (Ubuntu developers), provide this information to upstreams, work with them to upgrade their packages (for the ones we care about, roughly limited to packages in main). We also work with Debian and other distros to collaborate on a Python-ecosystem-wide upgrade. We'll feed as much work back into Debian as possible, depending obviously on their release cycle constraints.

https://code.launchpad.net/~barry/+git/pydeps contains scripts used to populate and sync this PPA.

As of 2017-01-18, we are using this PPA for the Python 3.5 -> 3.6 transition, and we are in step 2.

The long-term plan is to ship Ubuntu 18.04 with only Python 3.6.