pip 10 will break pybind11

[phdowling]

See: pypa/pip#3696 (comment)

It seems that the usage pip.locations.distutils_scheme('pybind11')['headers'] will no longer be supported in the upcoming pip upgrade (and in fact, is discouraged even in the current version of pip).

[wjakob]

That's quite concerning -- thank you for letting us know. A proposed fix is in #1190.