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/tests/parser/subscripts.py

@@ -0,0 +1,21 @@
+a[b]
+
+c[d,e]
+
+c1[d1,]
+
+# And now with many comments
+
+e[
+    # comment1
+    f
+    # comment2
+]
+
+g[
+    # comment3
+    h,
+    # comment4
+    i
+    # comment5
+]