Include the opening brace's line in multiline f-string errors

An unclosed f-string replacement field can report an error on a later line, making the opening brace hard to find:

var = "abc"
a = f""" {var} extern "C" { """
b = """ string """

  File "script.py", line 3
    b = """ string """
                   ^
SyntaxError: f-string: expecting '}'

The quotes after the second { start a string inside the replacement field. That string ends on line 3, and the final quotes start another string that is never closed.

The error should include the line number of the opening brace:

SyntaxError: f-string: expecting '}' to close '{' on line 2

The same problem affects t-strings. Valid strings inside replacement fields should continue to work.

Linked PRs

• gh-157779

从提供的多行 f-string 复现代码开始，并将其诊断结果与预期消息进行比较。检查对应的 t-string 情况，并确认替换字段中的有效字符串仍然可以正常工作。完成的标准是：两个受影响的错误消息都指出左花括号所在的行，且不会导致有效字符串回归；一个已关联的 pull request 已经列出。