Here is a relevant comment inside the KeyError_str function:
    /* If args is a tuple of exactly one item, apply repr to args[0].
       This is done so that e.g. the exception raised by {}[''] prints
         KeyError: ''
       rather than the confusing
         KeyError
       alone.  The downside is that if KeyError is raised with an
explanatory
       string, that string will be displayed in quotes.  Too bad.
       If args is anything else, use the default BaseException__str__().
    */

Why is it so important to round trip?