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I have no actual experience stargazing in France, but nobody answered, so I'll try.
Here's a picture of most of France at night: Source: www.nightearth.com
The dark areas have minimal light polution, and a relatively dark area is seen south of Paris. By looking at Google Maps, this seems like the area between OrlΓ©ans and Vierzon, on either side of the A71 road.
According to about-france.com this area is called La Sologne, and is popular with hunters, nature-lovers and hikers.
According to Google Maps, driving time from Paris to the area is about 1:45 hours.
Of course, you'll need to find a good spot in there - away from lights and without trees blocking the view, and where spending the night is possible and legal. Good luck.