Raise exception if BIDSDataGrabber is imported using Python 2

[adelavega]

Pybids no longer supports Python 2. Thus, BIDSDataGrabber should only be importable with Python 3, and raise an exception otherwise.

[adelavega]

I'd assign myself but I can't ;)

[oesteban]

I'd also include here setting the appropriate skips on tests:

• Because Python 2
• Because pybids was not installed in editable mode (very likely in user settings).

[adelavega]

Cool. Looks like Python 2 support might continue enough to the point that perhaps raise an Exception is a bit heavy, but it probably makes sense to skip on tests either way unless pybids CI adds back official python 2 support.

[effigies]

Is this actually a problem? Does BIDSDataGrabber depend on behavior since pybids dropped Python 2 support?

If not, can we set the Python 2 docker image to use the last Py2-compatible release tag?

So this:

https://github.com/nipy/nipype/blob/master/Dockerfile#L90-L91

Becomes

RUN curl -sSL https://github.com/INCF/pybids/archive/$TAG.tar.gz | tar xvz &&
    cd pybids && python setup.py develop

And you can set TAG to 0.3.0 for Py2, or master for current behavior.

[adelavega]

It doesn't now, but it probably will in the future, when additional functionality such as indexing derivatives is added. Feels a bit weird to me for BIDSDataGrabber to potentially behave differently based on your Python version.