Include step info in str(scroll_event).

Although event.step is only nonzero for scroll events, it seems
reasonable to always add it to str(MouseEvent).  After all, that str()
always contains e.g. dblclick, which doesn't make sense for
motion_notify_event either.  (IOW the alternative would be to more
carefully write different str()s for each kind of MouseEvents.)

Author: anntzer

lib/matplotlib/backend_bases.py

@@ -1433,7 +1433,7 @@ def _from_ax_coords(cls, name, ax, xy, *args, **kwargs):
     def __str__(self):
         return (f"{self.name}: "
                 f"xy=({self.x}, {self.y}) xydata=({self.xdata}, {self.ydata}) "
-                f"button={self.button} dblclick={self.dblclick} "
+                f"button={self.button} dblclick={self.dblclick} step={self.step} "
                 f"inaxes={self.inaxes}")