LONDON, March 3 (Reuters) – A unique artwork by the enigmatic street artist Banksy, personally painted and currently owned by American musician Mark Hoppus, is set to go under the hammer this Tuesday. The estimated auction price could reach as high as $6.35 million. The piece, titled "Crude Oil (Vettriano)," offers Banksy’s reinterpretation of "The Singing Butler," a 1992 painting by Scottish artist Jack Vettriano, whose recent passing was confirmed on Monday. While the