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

e179532284 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

mlir/test/python/dialects/arith_dialect.py

@@ -0,0 +1,19 @@
+# RUN: %PYTHON %s | FileCheck %s
+
+from mlir.ir import *
+import mlir.dialects.func as func
+import mlir.dialects.arith as arith
+
+def run(f):
+  print("\nTEST:", f.__name__)
+  f()
+
+# CHECK-LABEL: TEST: testConstantOp
+@run
+def testConstantOps():
+  with Context() as ctx, Location.unknown():
+    module = Module.create()
+    with InsertionPoint(module.body):
+      arith.ConstantOp(value=42.42, result=F32Type.get())
+    # CHECK:         %cst = arith.constant 4.242000e+01 : f32
+    print(module)