Python: Allow list_splats as type annotations

That is, the `*T` in `def foo(*args : *T): ...`.

This is apparently a piece of syntax we did not support correctly until
now.

In terms of the grammar, we simply add `list_splat` as a possible
alternative for `type` (which could previously only be an `expression`).
We also update `python.tsg` to not specify `expression` those places (as
the relevant stanzas will then not work for `list_splat`s).

This syntax is not supported by the old parser, hence we only add a new
parser test for it.

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

@@ -963,7 +963,7 @@ module.exports = grammar({
       field('type', $.type)
     )),
 
-    type: $ => $.expression,
+    type: $ => choice($.list_splat, $.expression),
 
     keyword_argument: $ => seq(
       field('name', choice($.identifier, $.keyword_identifier)),