Return a repr() for a dict keyed by a string, in sorted key order

def _stable_dict_repr(strdict):
    """Return a repr() for a dict keyed by a string, in sorted key order"""
    lines = []
    for key in sorted(strdict.keys()):
        lines.append("'%s': %s" % (key, rpr(strdict[key])))
    return "{%s}" % ", ".join(lines)