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/python.tsg

@@ -3169,11 +3169,11 @@
             (typed_parameter
                 (identifier) @name
                 .
-                type: (type (expression) @type)
+                type: (type (_) @type)
             )
             (typed_default_parameter
                 name: (_) @name
-                type: (type (expression) @type)
+                type: (type (_) @type)
                 value: (_) @value
             )
         ] @param
@@ -3228,7 +3228,7 @@
             (list_splat_pattern vararg: (_) @name) @starred
             (typed_parameter
                 (list_splat_pattern vararg: (_) @name) @starred
-                type: (type (expression) @type)
+                type: (type (_) @type)
             )
         ]
     ) @params
@@ -3245,7 +3245,7 @@
 
 ; Return type
 (function_definition
-    return_type: (type (expression) @type)
+    return_type: (type (_) @type)
 ) @funcdef
 {
     attr (@funcdef.funcexpr) returns = @type.node
@@ -3259,7 +3259,7 @@
             (dictionary_splat_pattern kwarg: (identifier) @name)
             (typed_parameter
                 (dictionary_splat_pattern kwarg: (identifier) @name)
-                type: (type (expression) @type)
+                type: (type (_) @type)
             )
         ]
     ) @params

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)),