BUG: Fix PyType_FastSubclass() check for numpy.int_

Signed-off-by: Yury V. Zaytsev <yury@shurup.com>
BUG: Document that numpy.int_ doesn't inherit from int on Py3k

Signed-off-by: Yury V. Zaytsev <yury@shurup.com>
TST: Add regression test to check numpy.int_ inheritance on Py2/Py3

Signed-off-by: Yury V. Zaytsev <yury@shurup.com>