Fix MLIR Python binding for arith.constant after argument has been changed to an interface

e1795322 removed the Type field from attributes and
arith::ConstantOp argument is now a TypedAttrInterface which isn't
supported by the python generator.
This patch temporarily restore the functionality for arith.constant but
won't generalize: we need to work on the generator instead.

Differential Revision: https://reviews.llvm.org/D130878