I am about to build a new python lib and I was seeking information concerning packaging in Python.
I understand that "setup.py" is the script that controls everything. I wonder how to deal with it when there are external libraries in svn for instance. 
How to download automatically a given version from the repository using "setup.py" ?
I may not have understood the problem correctly.
For any additional dependencies, you mention them in setup.py as

install_requires=['module1 >= 1.3', 'module2 >=1.8.2']

When you use setuptools, easy_install oo pip, these external dependencies will get installed during setup, if required. These should also be available in package repositories for download.
setup.py and source control repository