Allow some unparenthesized syntactic structures in f-string expression part (#159)

Resolves #157, #158

test/test_fstring.py

@@ -60,6 +60,13 @@ def grammar():
 
         # a line continuation inside of an format spec
         'f"{123:.2\\\nf}"',
+
+        # some unparenthesized syntactic structures
+        'f"{*x,}"',
+        'f"{*x, *y}"',
+        'f"{x, *y}"',
+        'f"{*x, y}"',
+        'f"{x for x in [1]}"',
     ]
 )
 def test_valid(code, grammar):

test/test_python_errors.py

@@ -274,6 +274,8 @@ def test_paren_kwarg():
         r'fr"\""',
         r'fr"\\\""',
         r"print(f'Some {x:.2f} and some {y}')",
+        # Unparenthesized yield expression
+        'def foo(): return f"{yield 1}"',
     ]
 )
 def test_valid_fstrings(code):