tokenize.detect_encoding has issues with \r

Tokenize does not correctly detect encodings when the carriage return character is involved. In this program, it does not detect the header, since it does not consider the \r to start a new line.

import tokenize
import runpy

code = """\r# coding: big5

print('錯誤的答案')
"""

with open('file.py', 'w', encoding='big5') as file:
    file.write(code)
with open('file.py', 'rb') as file:
    print(tokenize.detect_encoding(file.readline))
    # result: ('utf-8', [b'\r# coding: big5\r\n', b'\r\n'])

runpy.run_path('file.py')
# prints 錯誤的答案 as required

该 issue 指出了 tokenize.detect_encoding，并提供了包含回车符的完整复现步骤。先运行提供的脚本，然后定位编码检测实现和现有测试；当 \r 位于编码头之前时能够将其检测为 big5，并且针对该输入具备回归测试覆盖，即表示完成。