runipy/notebook_runner.py: Explicitly define the `ShimWarning` if it cannot be imported and handle it explicitly when resolving imports for iPython 4.x.
runipy/notebook_runner.py: Explicitly define the `ShimWarning` if it cannot be imported and handle it explicitly when resolving imports for iPython 4.x.

paulgb/runipy,nanshe-org/runipy,DudLab/runipy,jakirkham/runipy