You can use Selenium with Firefox for solving the issue, but it may not be suitable in many cases as the browser pops up every-time you run the code. Another idea is to use a headless broswer like PhantomJS.
The best way for this is to use the mechanize library. Install mechanize via pip.
pip install mechanize
Then you can use the following code:
import mechanize
mb = mechanize.Browser()
mb.addheaders = [('User-agent', 'Mozilla/5.0 (X11; U; Linux i686; en-US; rv:1.9.0.1) Gecko/2008071615 Fedora/3.0.1-1.fc9 Firefox/3.0.1')]
mb.set_handle_robots(False)
url = "http://www.gallimard-jeunesse.fr/searchjeunesse/advanced/(order)/author?catalog[0]=1&SearchAction=1"
response = mb.open(url).read()
print response
It also provides option for sleep and executing scripts. You can read them in the documentation.