DEPS: Minimum numpy version for pandas 1.0

[datapythonista]

There has been some discussion about dropping numpy 1.13, and require 1.15 for pandas 1.0. See #29212 (comment)

I think that'd be nice to reduce our matrix of builds for the CI, other than that I'm not sure what advantages this will have in our code base.

Any opinions @pandas-dev/pandas-core ?

[alimcmaster1]

Related: #29486

[gfyoung]

Just from an operational perspective, I agree with the comments in #29212 that we should follow the NEP and bump to 1.15. I don't know what functional benefits pandas gains from this, but as long as there are not any negative side effects, I think I'm okay with this.

[jbrockmendel]

closeable?

[TomAugspurger]

Yeah, we're just following NEP-29 now.