Add a Suite3 option with Ubuntu Focal.

[nuclearsandwich]

This configuration option requires two recent changes in stdeb and
ros_release_python:

• Add support for Suite3 option to release into different suites based on python version. stdeb/stdeb#147 for Suite3 support from stdeb (not yet merged or released)
• Add include support for aptly backed bootstrap repository. ros_release_python#33 for --include support on the current bootstrap repository.

The option allows a different set of suites to be set for Python 3
releases allowing us to forestall releasing Python2 versions of these
packages for platforms where we don't plan to support Python 2.

[dirk-thomas]

ros-infrastructure/ros_release_python#33 has nothing to do with this change.

[nuclearsandwich]

ros-infrastructure/ros_release_python#33 has nothing to do with this change.

--include doesn't work at all, suite3 or no without that change. This PR won't have the intended effect without it. I updated the PR body to clarify the relationship.

[dirk-thomas]

--include doesn't work at all, suite3 or no without that change. This PR won't have the intended effect without it.

Yes, --include was broken. Using Suite3 or just Suite doesn't make any difference. Therefore these two changes are orthogonal.