pip.main has moved, breaking the example setup.py

[stuaxo]

pip.main doesn't exist in current pip, so the example in the README no longer works.

I'm experimenting with an alternate setup.py using install_requires -

# -*- coding: utf-8 -*-
from setuptools import setup

setup_kwargs=dict(
    name='cairo-jupyter',
    packages=['cairo_jupyter'],
    install_requires=["jupyter-pip"],
)

try:
    from jupyterpip import cmdclass
    setup_params['cmdclass'] = jupyterpip
except:
    cmdclass=setup

setup(**setup_kwargs)

Haven't actually landed it in my own codebase yet, but when I do I can make a PR if this looks good here.