Change ipywidgets version check to use __version__. Fixes #1571.

[apayne97]

Note that for earlier versions of ipywidgets, __version__ doesn't exist so you may have to be creative with checking

[apayne97]

Maybe you should do something like:

if getattr(ipywidgets, version_info, False):
    if ipywidgets.version_info[0] < 4:
        raise ImportError()

If the ipywidgets version is stored in __version__ instead, then you know that the version is greater than 4!

[apayne97]

Woo!

[clonker]

Test failures are unrelated, merging