Support Python 3.5

[cancan101]

Right now only python 3.6 is supported:

/usr/src/app/src/niwidgets/niwidgets/niwidget_volume.py in __init__(self, filename)
     41                     ``PosixPath`` from python3's pathlib.
     42         """
---> 43         self.filename = Path(filename).resolve(strict=True)
     44 
     45         # load data in advance

TypeError: resolve() got an unexpected keyword argument 'strict'

due to strict being added in 3.6.

[janfreyberg]

Ah wow, OK - didn't realise this. Thanks.