We want to default to running all tox environments under python 3, so
set the basepython value in each environment.
We do not want to specify a minor version number, because we do not
want to have to update the file every time we upgrade python.
We do not want to set the override once in testenv, because that
breaks the more specific versions used in default environments like
py35 and py36.
To install horizon, check python3 version and set target directory
in postinstall script on package.json.
Change-Id: I181b2a204f6998c24aa188b83a66c596cfb44f1c
Co-Authored-By: Shu Muto <shu.mutow@gmail.com>
cf27da5337