horizon plugins use horizon as a library.
Having horizon in upper-constraints would be helpful
to avoid unexpected breakage due to a newer horizon releases,
particularly in stable branches.
The reason we don't have horizon in upper-constraints so far is
because most horizon plugins would like to work and test with horizon
in a same branch, but Zuul v3 allows such tests.
This reasoning is no longer true.
Updated to stable/train maximum of 16.0.0.
Note that this is proposed to stable branches directly.
In the master branch, it is required to split out common portions
in horizon into a separate deliverable and it was rejected to
add horizon to upper-constraints.txt.