Clarify that format strings accept arbitrary tokens as field names

Bug report

Bug description:

Should the code below produce an error?

'{(}'.format(**{'(': 2})

This produces '2' in Python 3.14.5 on MacOS.

Reading
https://docs.python.org/3/library/string.html#format-string-syntax

replacement_field: "{" [field_name] ["!" conversion] [":" format_spec] "}"
field_name:        arg_name ("." attribute_name | "[" element_index "]")*
arg_name:          [identifier | digit+]

this ends up pointing to identifier at https://docs.python.org/3/reference/lexical_analysis.html#grammar-token-python-grammar-identifier

NAME:          name_start name_continue*
name_start:    "a"..."z" | "A"..."Z" | "_" | <non-ASCII character>
name_continue: name_start | "0"..."9"
identifier:    <NAME, except keywords>

Given that '(' is an Ascii character, then ( is not a valid identifier and it should not be allowed.

CPython versions tested on:

3.14

Operating systems tested on:

macOS

Linked PRs

• gh-151064
• gh-153143

首先复现所示的 format-string 示例，并阅读链接的 format string 语法和标识符语法文档。将文档中规定的字段名规则与观察到的行为进行比较，并将链接的 PRs 视为已有工作；当行为和文档就是否接受任意 token 达成一致时，即表示完成。