BUG: Fix use of setuptools.command.install._install.

This private variable has disappeared in later versions of setuptools.
In older versions of setuptools it is the same as
distutils.command.install.install, so use that instead.

Closes #4664.
Merge pull request #4667 from charris/fix-numpy-distutils

BUG: Fix use of setuptools.command.install._install.