retina matplotlib backend failing with execnb

[jph00]

The nbdev_test command failed with the following error when I had these two lines in the notebook.

%matplotlib inline
%config InlineBackend.figure_format = 'retina'

Error message of nbdev_test

While Executing Cell #6:
---------------------------------------------------------------------------
ModuleNotFoundError                       Traceback (most recent call last)
<ipython-input-1-0eb3525f3a9a> in <module>
----> 1 get_ipython().run_line_magic('matplotlib', 'inline')
      2 get_ipython().run_line_magic('config', "InlineBackend.figure_format = 'retina'")
      3
      4 import pandas as pd

~/Library/Python/3.7/lib/python/site-packages/IPython/core/interactiveshell.py in run_line_magic(self, magic_name, line, _stack_depth)
   2334                 kwargs['local_ns'] = self.get_local_scope(stack_depth)
   2335             with self.builtin_trap:
-> 2336                 result = fn(*args, **kwargs)
   2337             return result
   2338

<decorator-gen-101> in matplotlib(self, line)

~/Library/Python/3.7/lib/python/site-packages/IPython/core/magic.py in <lambda>(f, *a, **k)
    185     # but it's overkill for just that one bit of state.
    186     def magic_deco(arg):
--> 187         call = lambda f, *a, **k: f(*a, **k)
    188
    189         if callable(arg):

~/Library/Python/3.7/lib/python/site-packages/IPython/core/magics/pylab.py in matplotlib(self, line)
     97             print("Available matplotlib backends: %s" % backends_list)
     98         else:
---> 99             gui, backend = self.shell.enable_matplotlib(args.gui.lower() if isinstance(args.gui, str) else args.gui)
    100             self._show_matplotlib_backend(args.gui, backend)
    101

/usr/local/lib/python3.7/site-packages/execnb/shell.py in enable_matplotlib(self, gui)
     91     def enable_matplotlib(self, gui=None):
     92         "Enable `matplotlib` in a nested shell"
---> 93         from matplotlib_inline.backend_inline import configure_inline_support
     94         configure_inline_support.current_backend = 'unset'
     95         return super().enable_matplotlib(gui)

ModuleNotFoundError: No module named 'matplotlib_inline'

cc @seeM

[seeM]

I'm unable to reproduce this. seeM@662969d adds a notebook with the mentioned lines to a test branch. All tests pass in CI for that commit. cc @bkowshik @jph00