Correct typo in interp() docstring.

The docstring for interp() contained the grammatically-incorrect text "defaults is". I corrected this to "default is".
Merge pull request #4111 from chrisdembia/patch-1

Correct typo in interp() docstring.
MAINT: setup.py improvements - allow some setuptools commands to work.
Merge pull request #4110 from rgommers/setuppy

MAINT: setup.py improvements - allow some setuptools commands to work.