lookdict() is general-purpose, and may return NULL if (and only if) a
comparison raises an exception (this was new in Python 2.5).
lookdict_unicode() below is specialized to string keys, comparison of which can
-never raise an exception; that function can never return NULL.
+never raise an exception; that function must not return NULL for string key.
lookdict_unicode_nodummy is further specialized for string keys that cannot be
the <dummy> value.
For both, when the key isn't found a PyDictEntry* is returned