By Pamela Barbaglia LONDON (Reuters) – France’s Thales is working on a plan to buy the cybersecurity business of IT consultancy group Atos, sources told Reuters, in a potential $3 billion tie-up likely to test the political determination for shoring up France’s digital defences. Thales, which ranks as Europe’s largest defence electronics company, and its adviser Centerview Partners have approached several private equity firms including Bain Capital to explore a possible joint offer as part of a deal that would involve a complex break-up of Atos, the sources said. Thales would buy the big data …