A nested unpack target with attribute leaves did not parse

Vloot: (self.enc, self.enc_value), (self.dec, self.dec_value) = pair(...) failed with "expected expression" (the pin still does).

Fix: the nested-unpack records (PyNestAdd / PyParseNestGroupInto / PyNestEmit) take an ident.ident leaf in an assignment group. It is held under a hidden name and stored through PyMakeAttrStore. PySkipTargetGroup steps over it the same way.

Test: test_nilpy_nested_unpack_into_attributes.