When using a file:// in a RefResolver to resolve relative URI references it fails with:

jsonschema.exceptions.RefResolutionError: addinfourl instance has no attribute '__exit__'

This is because urllib.urlopen() can't be used in a with clause - see https://stackoverflow.com/questions/30627937/traceback-attributeerroraddinfourl-instance-has-no-attribute-exit for details.

I can create a minimal example if needed, and I'll submit a pull request for the fix.