How to use python 3 as a build script in non-python travis configuration?

If you want to use the container based infrastructure you can make use of the apt addon:

addons:
  apt:
    sources:
      - deadsnakes # source required so it finds the package definition below
    packages:
      - python3.5

Packages that can be used are listed here

Update

In order to use dependencies with c-extensions like psycopg2 or pyYAML it is also necessary to add python3.4-dev