RFC: Make the new API and traitlets work PY3 only

[satra]

Several python projects are moving over to PY3 only. Given the significant changes coming our way with the new API and transition to traitlets, I would like to suggest that:

1. [api 1] we release 1.0 by end of summer (Sep 1) and move it purely into a bug-fix mode. we will allow enhancements till then, but recommend that people start looking at 2.0.
2. [upgrade option] we will augment our export tool so that an exported workflow can be imported into PY3 version.
3. [api 2] we create a development branch supporting only PY3, do all traitlests and API development here, and get rid of future. this becomes our main development target.
4. the development branch won't becomes default when the new API and traitlets are in place.
5. we target releasing 2.0 by end of the year

@chrisfilo, @oesteban, @djarecka, @effigies, @hjmjohnson, @mih, @yarikoptic, @TheChymera, @mgxd - any thoughts here?

[oesteban]

Sounds very good!

[chrisgorgo]

+1 for dropping PY2 support

[oesteban]

@satra what about a new "nipype" github organization?

• nipype/nipype would contain the bleeding edge development (2.0)
• nipype/nipype-1.0 would maintain the legacy version where we only include bugfixes

We could/should split the workflows (nipype/workflows?), making altogether a lot more manageable. Plus, most of the people just want nipype and start playing around the example workflows. I don't think it will make much difference for the users, but will make the projects nicer to maintain.

[TheChymera]

When are the deps scheduled to drop py2 support? It comes a bit as a surprise to me that this is already happening. Meanwhile, other parts of the pythosphere, e.g. PsychoPy - which I assume should be highly relevant to human neuroimagers - are still not fully supporting a py3 environment.

[satra]

@TheChymera - the current api will continue to support py2 and 3 as is the case now, but will be frozen as of the 1.0 release. the new api release 2.0 targeted later in the year will be py3 only. python itself is no longer doing anything with py 2.x.

@oesteban - would be interesting to consider an organization, let's chat during ohbm. at present i do want to preserve history, so we will create a separate branch i suspect we will re-use a lot of things from the current interfaces.