Backport PR #9299: FIX: account for dpi == 'figure'

In mpl 1.5.0 'figure' became a valid dpi value, in 2.0 it will
become the default.

xref https://github.com/matplotlib/matplotlib/issues/6113

...

IPython/core/pylabtools.py

@@ -98,6 +98,8 b" def print_figure(fig, fmt='png', bbox_inches='tight', **kwargs):"
         return
 
     dpi = rcParams['savefig.dpi']
+    if dpi == 'figure':
+        dpi = fig.dpi
     if fmt == 'retina':
         dpi = dpi * 2
         fmt = 'png'