It's illegal for a bank to refuse you as a customer, even if you are illegally on french territory for example. If a bank do so, you can call the Banque de France that will enroll you in a random french bank.
Although it's illegal, lots of banks try to trick people, especially poor one, students, strangers, people who get debts, etc.
If you don't want or don't have time to enforce your rights, you can simply ask a friend or a fellow student to write an attestation d'hébergement - proof of accommodation - for you , just as if you were living at the guy place for free. This hand-written letter must mention that the guy gives you a couch à titre gracieux (for free).
You will also need an electricity bill from the guy and his identity card copy. This is a common practice. I do this many times with almost-homeless-friends.
Assuming you don't want to stay at hostel, it's pretty easy to find roommates among PhD people, even for short times. People sometimes go away from the flat for one week, one or two months and, as rooms are very expensive in Paris, they are happy to sub-locate it to you.
Some people that can help you :
If you are a Maths PhD guy, I'm astonished it would be a problem. You should maybe try again in another bank just as la Banque Postale wich once, was a popular bank, even if some seems to have bad practices.
Last (important) advice: if you are in Paris, choose a bank from a traditionally "poor" arrondissement, like 13ème, 18ème, 19ème or even suburb, Montreuil or Bagnolet are very good choices (east suburb, metro 9 and 11).