[Python] Add PyDecimal_Check(pythopn_decimal) as a ARROW_DCHECK

Author: HyukjinKwon

python/pyarrow/src/arrow/python/decimal.cc

@@ -51,9 +51,9 @@ static Status InferDecimalPrecisionAndScale(PyObject* python_decimal, int32_t* p
   ARROW_DCHECK_NE(python_decimal, NULLPTR);
   ARROW_DCHECK_NE(precision, NULLPTR);
   ARROW_DCHECK_NE(scale, NULLPTR);
+  ARROW_DCHECK(PyDecimal_Check(python_decimal))
+      << "python_decimal is not an instance of decimal.Decimal";
 
-  // TODO(phillipc): Make sure we perform PyDecimal_Check(python_decimal) as a
-  // ARROW_DCHECK
   OwnedRef as_tuple(PyObject_CallMethod(python_decimal, const_cast<char*>("as_tuple"),
                                         const_cast<char*>("")));
   RETURN_IF_PYERROR();