Breaking changes:

* ``common`` role:

  * Added separate parameter (``pip_check_requirements_py3``) for

    specifying dedicated Python 3 virtual environment package

    requirements used for package upgrade checks on (other

    user-provided) Python 3 virtual environments. If the existing

    ``pip_check_requirements`` parameter has been overridden, the new

    parameter will most likely need to be overridden in your site

    configuration as well. Take note that the new requirements will

    differ between Debian Jessie and Debian Stretch due to differnece

    in Python 3 minor version releases.