为什么Python3会得到一个读取文本文件的UnicodeDecodeError，而Python2不会？

import chardet

def read_file(filename):
    try:
        with open(filename, 'r', encoding='utf-8') as file:
            content = file.read()
            print("File read successfully with UTF-8 encoding.")
            return content
    except UnicodeDecodeError:
        print("Error decoding file with UTF-8 encoding. Trying to detect encoding...")
        with open(filename, 'rb') as file:
            raw_data = file.read()
            result = chardet.detect(raw_data)
            encoding = result['encoding']
            content = raw_data.decode(encoding)
            print(f"File read successfully with detected encoding: {encoding}")
            return content

# 示例调用
content = read_file('filename.txt')
print(content)