Use python_requires to check the python version so py2.py3 wheels are not built and pip will work correctly.

### Problem



* The wheels being built for ipywidgets claim it is python 2 compatible. eg ipywidgets-7.6.3-py2.py3-none-any.whl

* Modern versions of pip do not understand that ipywidgets is not python 2 compatible. setup.py is not run when downloading a wheel.



So it installs it without error on python 2.



I believe the  py2.py3 wheels are being built because the _python_requires_ argument is not being passed to setup but I do not know that for sure.



### Proposed Solution



Add a python_requires argument to the setup call to solve it.



EG Add this to [setup_args in setup.py](https://github.com/jupyter-widgets/ipywidgets/blob/90c1c14211a6be685c6077646f40973f96faced1/setup.py#L67) 



```python

python_requires = '>=3.5',

```



Then you can remove this

```python

#-----------------------------------------------------------------------------

# Minimal Python version sanity check

#-----------------------------------------------------------------------------



import sys



v = sys.version_info

if v[:2] < (3, 5):

    error = "ERROR: %s requires Python version 3.5 or above." % name

    print(error, file=sys.stderr)

    sys.exit(1)



```



This issue from another project has a very good description of the problem and how to solve it



https://github.com/PLOS/allofplos/issues/20

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