Use os.path.join properly in setup.py

[rs2]

Else fails on Windows:

     with open(os.path.join(HERE, 'urth/widgets/ext/_version.py')) as f:
 FileNotFoundError: [Errno 2] No such file or directory: 'C:\\Users\\foobar\\AppData\\Local\\Temp\\pip-temp-build\\urth/widgets/ext/_version.py'

https://github.com/jupyter-incubator/declarativewidgets/blob/master/setup.py#L13

Leave this to me, I'll test and submit a PR.

[lbustelo]

@rs2 Cool... thanks. I'll keep an eye out for the PR.