python-click

Have you checked Synaptic Package Manager?

I did. There is a "python3-click" package but it's totally unrelated to click (the CLI creation library made by Werkzeug's developer).

$ apt-cache search python-werkzeug
python-werkzeug - collection of utilities for WSGI applications
python-werkzeug-doc - documentation for the werkzeug Python library

As you can see Werkzeug is available. But click gives an empty result:

$ apt-cache search python-click

I also searched on the web but I don't find it on Ubuntu repositories.

The ubuntu package repositories do not contain all the python libraries. They contain many important libs and the onces other Ubuntu packages rely on.
If you are developing in Python you should consider using virtualenv (http://virtualenv.readthedocs.org/en/latest/) and pip.

Thanks for the answer. I need click to be automatically installed as a dependency for a deb package. Do you know how I can achieve this? (apart from depending on "python-pip" and calling "sudo pip install click" manually..)

Ok I think I found a "workaround": http://stackoverflow.com/questions/12210389/how-to-add-pypi-dependencies-to-deb-package

Still, where should I look in order to "suggest" adding something to Ubuntu repositories? Is it possible to do so?

Well, that is not the way it works. If you have a "private" dependency, you have to add your dependency packages to a private repository or distribute them in an other way. The official repositories are not there to satisfy every need.
You have the option to use a PPA for open source software, If your main software is open source you can also publish it that way. Else you can use it for the dependencies.