debugshell: disable demand importer when importing debugger For reasons I can't explain (but likely have something to do with a combination of __import__ inferring default values for arguments and the demand importer mechanism further assuming defaults), the demand importer isn't playing well with IPython. Without this patch, we get a failure "ValueError: Attempted relative import in non-package" when attempting to import "IPython." The stack has numerous demandimport calls on it and adding "IPython" to the exclude list in demandimport isn't enough to make the problem go away, which means the issue is likely somewhere in the bowells of IPython. It's easier to just disable the demand importer when importing the debugger.