Introduce release candidates

[obestwalter]

Instead of providing builds for testing via https://devpi.net which need to be explicitly installed I would like to start uploading release candidates to pypi instead. ~~~Ideally that would be just one before a release that is online for a week or two and then the final release is uploaded.~~~ (actually not quite sure about this).

The main advantage from my point of view would be that users can persistently opt-in to these in their CI set ups by using e.g. pip install tox>=2.8rc1 (pip will then always install pre release versions, also for other releases - see pypa/pip#1545).

I think this would make the preparation of releases smoother and we have a chance to catch problems earlier, because there is a subset of users who are willing to live on the bleading edge :)

[obestwalter]

I think I will try this for the 2.8 release then. At the weekend there is a sprint at EuroPython and we'll prepare a 2.8.0.rc1 and release it.

[obestwalter]

Well ... life and such. Took a bit longer, but this weekend the rc will go out and this needs no longer be open.

[obestwalter]

to quote myself:

We used rcs ant they did not save as from the bug we introduced. We fixed that bug quite quickly though ober the slow CI weekend :)

I think for now, that rc are an unnecessary complication and that it is better to release much more often and fix bugs quickly, when they pop up.

[obestwalter]

On further thought - maybe giving up on it too quickly is not the right way to go either. I think I will still do an rc and retag it after a few days to be a normal release for a while ...