"Roe v. Wade, 410 U. S. 113 (1973), held that women seeking abortions fell into the mootness exception for cases “ ‘capable of repetition, yet seeking review,’ ” enabling them to sue after they terminated their pregnancies without showing that they intended to become pregnant and seek an abortion again. Id., at 125. Yet, since Singleton, the Court has unquestioningly accepted doctors’ and clinics’ vicarious assertion of the constitutional rights of hypothetical patients, even as women seeking abortions have successfully and repeatedly asserted their own rights before this Court."