virtualenv for toaster and python 3

[Richard Purdie <richard.purdie@linuxfoundation.org>]

The move to python3 is unsurprisingly causing some issues in places.
One of them is unfortunately the use of vritualenv, which as I
understand it, toaster currently promotes as its preferred way of being
run.

The issue is that we need "python" to be python v2 but "python3" to be
python v3. It seems with virtualenv you can either have python v2 or
python v3 but not both.

The python developers official party line is that "python" is v2 and
"python3" is v3. There are some distros choosing not to do this (e.g.
Arch Linux) but most are following the official recommendation and I
think OE needs to follow the official recommendation too. Its likely OE
will need to error with sanity test failures if the environment doesn't
conform to this view of the world.

Why do we need this? We have a mixture of code, some of it is ported to
v3, some isn't. Whilst we're aiming to convert all of "our" code to
python v3, even once we've done that, we can't easily change scripts
that ship with the source code we build for example, or expect everyone
to do that for all software they build.

It may be virtualenv can be configured to not take over "python" but
only "python3" but Ed/I haven't found the option, I'm no expert.

The other alternative is to promote the use of something like "pip3
install 'Django>1.8,<1.9' --user" instead. Toaster doesn't need many
modules and those it does need should be installable into the user's
homedir using pip3.

Whilst this means changing the documentation and taking a new approach,
I think its the "least bad" solution we have right now.

Obviously I'm open to other ideas but we really do need a way which
allows mixed versions of python and virtualenv doesn't seem to work for
this.

Cheers,

Richard