“ Intervention by the federal courts in state elections has always been a serious business. Here the city has already incurred considerable expense in preparing for an election to be held within the next three weeks. If this election were held, applicants could later bring suit to have it set aside. I thus do not see why these plans should be stopped in midstream in a case in which the legal issues are unclear, when the election cannot result in the severe irreparable harm necessary to justify the issuance of the extraordinary remedy of an injunction by an individual Justice. ”