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,))

python/extractor/tsg-python/python.tsg

@@ -3485,5 +3485,9 @@
 
 [(tuple element: (_)) (tuple_pattern)] @tup
 {
-    attr (@tup.node) parenthesised = #true
+    ; In order to avoid writing to the `parenthesised` attribute twice, we only set it here
+    ; if the surrounding expression is not a `parenthesized_expression`.
+    if (not (instance-of (get-parent @tup) "parenthesized_expression")) {
+        attr (@tup.node) parenthesised = #true
+    }
 }