Japanese carrier SoftBank is buying a 70% interest in U.S. carrier Sprint for $20.1 billion, as announced at a Tokyo press event. The deal is expected to close in mid-2013, during which time SoftBank will purchase $8 billion in new shares and $12.1 billion in existing shares from Sprint. The two companies will have the third-highest mobile service revenue of any company, and the combined subscriber base will be one of the world’s largest.…