Function lookdict_string

* Hacked up version of lookdict which can assume keys are always strings; * this assumption allows testing for errors during PyObject_RichCompareBool() * to be dropped; string-string comparisons never raise exceptions. This also * means we don't need to go through PyObject_RichCompareBool(); we can always * use _PyString_Eq() directly. * * This is valuable because dicts with only string key