allow tox-pyenv to install missing python interpretors

[ssbarnea]

At this moment there are manual steps one should take in order to install all python environments needed for testing using tox-pyenv. The project homepage specifies on how to install these on CircleCI on the home page.

How about if we can automatically install missing python interpreters so a simple execution of tox would install them when they are missing.

This would make testing much easier on various CI systems (Travis, Jenkins) and even development environment.

If there are concerns regarding change of behaviour we could make this optional by adding an option named install_missing_interpreters = true to tox.ini.

[stavxyz]

@ssbarnea I like this idea. I have an open WIP to "enable" missing interpreters:

https://github.com/samstav/tox-pyenv/pull/4

which should be adequate for most cases, esp. in CircleCI, since they have so many python version s (pretty much all of them?) installed already, and I think this feature in #4 would be safe to enable by default.

I think having download/install available would be great, but I would probably prefer to have it disabled by default.

Thoughts?

[ZaxR]

One implementation that could be borrowed from: https://github.com/pojx/tox-pyenv-install