Add missing default constructor for stride iterator (#327)

It's a requirement for being adapted by std::reverse_iterator

Author: serge-sans-paille

include/xtensor-python/pystrides_adaptor.hpp

@@ -89,6 +89,8 @@ namespace xt
         using iterator_category = std::random_access_iterator_tag;
         using shape_pointer = typename pystrides_adaptor<N>::shape_type;
 
+        pystrides_iterator() = default;
+
         inline pystrides_iterator(pointer current, shape_pointer shape)
             : p_current(current)
             , p_shape(shape)