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

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