DEPR: MultiIndex.to_hierarchical, labels
Looks like pyarrow is still using the deprecated usage.  We aren't pinning the version, and it looks like the py36_macos azure build is using 0.9.0 (which is also what we have listed in `compat._optional`).  Testing locally, it looks like 0.12.0 is the first minor release that doesn't usage the deprecated usage.

Anyone mind bumping the supported pyarrow min version to 0.12.0?  cc @jorisvandenbossche  ok with bumping pyarrow bumped pyarrow min version to 0.12.0 and updated usages The azure mac-py36 build has numpy=1.13.3 pinned but pyarrow0.12.0 requires numpy>= 1.14.0

Should we bump the numpy pin in this build or remove pyarrow? or we could just bump the numpy min version.  1.13.3 was released 2017-09-29 we should bump numpy anyhow to 1.14/1.15

let’ for now bumped the pin in this build to 1.14, can bump the numpy min ver in a dedicated PR Bumping pyarrow to 0.12 is certainly OK I think! thanks, 2 small followons.