MNT: ensure we can show un-pickled figures

For Matplotlib < 3.2 inline figures maybe re-hydrated with the canvas
attribute as None.  In that case `print_figure` will fail, this ensure
that a minimal canvas will exist.

IPython/core/pylabtools.py

@@ -125,6 +125,10 b" def print_figure(fig, fmt='png', bbox_inches='tight', **kwargs):"
     kw.update(kwargs)
     
     bytes_io = BytesIO()
+    if fig.canvas is None:
+        from matplotlib.backend_bases import FigureCanvasBase
+        FigureCanvasBase(fig)
+
     fig.canvas.print_figure(bytes_io, **kw)
     data = bytes_io.getvalue()
     if fmt == 'svg':

IPython/core/tests/test_pylabtools.py

@@ -248,3 +248,9 b' class TestPylabSwitch(object):'
 def test_no_gui_backends():
     for k in ['agg', 'svg', 'pdf', 'ps']:
         assert k not in pt.backend2gui
+
+
+def test_figure_no_canvas():
+    fig = Figure()
+    fig.canvas = None
+    pt.print_figure(fig)