BLD: add numpy requirement to metadata if it can't be imported.

This should help Buildout users.  Closes gh-3379.

(cherry picked from commit 8e7ee0c4b3c16e3e81d2298d40ee8373feb65a2d)
BLD: add minimum required numpy version to build_requires if numpy not found.

(cherry picked from commit 9b626bea935f95b2d3a7b1c37ab30610d8671440)