bpo-40176: Improve error messages for trailing comma on from import (GH-20294)

Author: isidentical

Grammar/python.gram

@@ -134,8 +134,9 @@ import_from[stmt_ty]:
         _Py_ImportFrom(NULL, b, _PyPegen_seq_count_dots(a), EXTRA) }
 import_from_targets[asdl_seq*]:
     | '(' a=import_from_as_names [','] ')' { a }
-    | import_from_as_names
+    | import_from_as_names !','
     | '*' { _PyPegen_singleton_seq(p, CHECK(_PyPegen_alias_for_star(p))) }
+    | invalid_import_from_targets
 import_from_as_names[asdl_seq*]:
     | a=','.import_from_as_name+ { a }
 import_from_as_name[alias_ty]:
@@ -670,3 +671,6 @@ invalid_double_type_comments:
 invalid_del_target:
     | a=star_expression &del_target_end {
         RAISE_SYNTAX_ERROR_KNOWN_LOCATION(a, "cannot delete %s", _PyPegen_get_expr_name(a)) }
+invalid_import_from_targets:
+    | import_from_as_names ',' {
+        RAISE_SYNTAX_ERROR("trailing comma not allowed without surrounding parentheses") }

Lib/test/test_syntax.py

@@ -641,6 +641,14 @@
 Traceback (most recent call last):
 SyntaxError: cannot assign to f-string expression
 
+>>> from t import x,
+Traceback (most recent call last):
+SyntaxError: trailing comma not allowed without surrounding parentheses
+
+>>> from t import x,y,
+Traceback (most recent call last):
+SyntaxError: trailing comma not allowed without surrounding parentheses
+
 Corner-cases that used to fail to raise the correct error:
 
     >>> def f(*, x=lambda __debug__:0): pass