[3.9] bpo-41659: Disallow curly brace directly after primary (GH-22996) (#23006)

(cherry picked from commit 15acc4eaba8519d7d5f2acaffde65446b44dcf79)
diff --git a/Lib/test/test_syntax.py b/Lib/test/test_syntax.py
index d6583b9..b0527e6 100644
--- a/Lib/test/test_syntax.py
+++ b/Lib/test/test_syntax.py
@@ -802,6 +802,9 @@
         else:
             self.fail("compile() did not raise SyntaxError")
 
+    def test_curly_brace_after_primary_raises_immediately(self):
+        self._check_error("f{", "invalid syntax", mode="single")
+
     def test_assign_call(self):
         self._check_error("f() = 1", "assign")