Python: Allow comments in subscripts

Once again, the interaction between anchors and extras (specifically
comments) was causing trouble.

The root of the problem was the fact that in `a[b]`, we put `b` in the
`index` field of the subscript node, whereas in `a[b,c]`, we
additionally synthesize a `Tuple` node for `b,c` (which matches the
Python AST).

To fix this, we refactored the grammar slightly so as to make that tuple
explicit, such that a subscript node either contains a single expression
or the newly added tuple node. This greatly simplifies the logic.

python/extractor/tsg-python/tsp/grammar.js

@@ -929,11 +929,18 @@ module.exports = grammar({
       field('attribute', $.identifier)
     )),
 
+    _index_expression: $ => choice(
+      $.list_splat,
+      $.expression,
+      $.slice
+    ),
+
+    index_expression_list: $ => open_sequence(field('element', $._index_expression)),
+
     subscript: $ => prec(PREC.call, seq(
       field('value', $.primary_expression),
       '[',
-      commaSep1(field('subscript', choice($.list_splat, $.expression, $.slice))),
-      optional(','),
+      field('subscript', choice($._index_expression, $.index_expression_list)),
       ']'
     )),