Matplotlib: panel label out of the box, above the ylabel

Matplotlib: panel label out of the box, above the ylabel

I have been using matplotlib for all my publication-use figures, and one
question is: how to I place the panel labels that is out of the box? The
tutorial is shown here:

But the panel label is only at the corner of the bounding box, not out
side of the box and above the y axis label (which is the usual way), like
this:

Any input would be appreciated. Thanks!