change __str__ output similar to Numpy (#521)

Author: shssf

dpnp/dparray.pyx

@@ -193,10 +193,7 @@ cdef class dparray:
 
         """
 
-        for i in range(self.size):
-            print(self[i], end=' ')
-
-        return "<__str__ TODO>"
+        return str(numpy.asarray(self))
 
     # The definition order of attributes and methods are borrowed from the
     # order of documentation at the following NumPy document.