Python: Fix parenthesized tuple parser bug

We were writing the `parenthesised` attribute twice on tuples, once
because of the explicit parenthetisation, and once because all non-empty
tuples are parenthesised. This made `tree-sitter-graph` unhappy.

To fix this, we now explicitly check whether a tuple is already
parenthesised, and do nothing if that is the case.

python/extractor/tests/parser/collections.py

@@ -35,3 +35,5 @@ t, u,
   x, y,
 #comment
 )
+
+((z,))