allow specification of build-time dependencies on egg_info invokation

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This is on the roadmap but I couldn't find a specific issue for this, so here it is

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related: pypa/setuptools#917

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@takluyver

If you have time, please implement the following behavior:

try:
    from setuptools.pep517 import get_build_requires
    requirements = get_build_requires({})
    # Install setup requirements
except Exception as e:
     print(e)

This will comply with pypa/setuptools#1039 and will attempt to install build requirements, but will fail gracefully if unable. It also complies with PEP 517. I think pip 10.0 should at least resolve this issue, even if the full PEP 517 isn't implemented.