ipykernel backend requires matplotlib>=2.0.0

As a result of this line:

https://github.com/ipython/ipykernel/blob/34ca17011c1e79f65290b88f1deb18ac03e142af/ipykernel/pylab/backend_inline.py#L195

the `inline` backend provided by `ipykernel` requires `matplotlib>=2.0.0`. However, since ipykernel doesn't declare its own dependency on matplotlib, its quite easy to install a jupyter notebook instance that doesn't work.

It is possible to put a matplotlib version check in the header for `ipykernel.pylab` so that errors are raised as early as possible (i.e. when using `%matplotlib inline` in ipython)?