BUG: Avoid type promotion in tril and triu.
MAINT: disable ufunc override for 1.9 release

The ufunc override `__numpy_ufunc__` still has a few unresolved issues
regarding its behavior towards Python operators see gh-4815.
To avoid releasing numpy with an interface that might change in the next
numpy version and to not further delay the 1.9 release it has been
decided to disable the feature in 1.9.0.
Merge pull request #4920 from juliantaylor/ufunc-disable

disable ufunc override for 1.9 release