Python: Fix parsing of await inside expressions

Found when parsing `Lib/test/test_coroutines.py` using the new parser.

For whatever reason, having `await` be an `expression` (with an argument
of the same kind) resulted in a bad parse. Consulting the official
grammar, we see that `await` should actually be a `primary_expression`
instead. This is also more in line with the other unary operators, whose
precedence is shared by the `await` syntax.

Author: tausbn

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

@@ -751,14 +751,14 @@ module.exports = grammar({
       $.comparison_operator,
       $.not_operator,
       $.boolean_operator,
-      $.await,
       $.lambda,
       $.primary_expression,
       $.conditional_expression,
       $.named_expression
     ),
 
     primary_expression: $ => choice(
+      $.await,
       $.binary_operator,
       $.identifier,
       $.keyword_identifier,
@@ -1202,7 +1202,7 @@ module.exports = grammar({
 
     await: $ => prec(PREC.unary, seq(
       'await',
-      $.expression
+      $.primary_expression
     )),
 
     comment: $ => token(seq('#', /.*/)),